Food is still undercooked on cast iron

Your pan was too hot. Cast iron pans can get ripping hot (which is good) and retain heat very well (which is also good). But, on the other hand, if you have a thicker piece of meat and want medium doneness, you should not start with maximum heat, depending on your stove.

If your pan is really that hot that the outside looks burned while the inside is still raw, the heat from the pan simply did not have enough time to penetrate into the inner layers of your food.

A few approaches to your problem could be:

• take your food out of the fridge at least half an hour before throwing it into the pan, to allow it to get up in temperature - this does not make a big difference as per SirHawrks comment quoting Kenji Lopez-Alt
• start with lower heat and sear your food for longer
• start with high heat to get a good searing, then transfer the whole pan to a pre-heated oven and finish cooking in there
• use a meat thermometer to gauge the doneness of the interior

The very short answer: You had bad temperature control.

You have to leave meat on the skillet until the proper internal temperature is reached. If the outside burns before that, then you used too high heat. Also, if you have a very thick steak, you may need to use more involved methods.

A longer answer: It is absolutely normal that cast iron behaves very differently from a typical thin nonstick pan. This is why people are making such a big deal out of the kind of pan they are using - cooking with both is simply different.

When transitioning to using the cast iron, you have to take into account that it has a much better heat transfer into the food, but also reacts a lot slower to changes in the burner setting. It also manages to make much better crusts, I suspect that this is because it stays very hot in the places where it makes contact with the food, as opposed to an aluminum pan where the heat gradient might extend a bit into the pan itself.

So you will need to learn to recognize when the pan is at the proper "temperature" (actually, the proper rate of heat transfer) and use a burner setting that provides that temperature and not a higher or lower one. The smoking oil is not a good indicator, because the smoking oil is only an indicator of the current temperature at the current moment, not of the rate of heat transfer at equilibrium. You will simply have to experiment until you have found the right setting for your burner, pan and usual meat thickness. You also have to curb your impatience and give it enough time to reach that temperature at the needed setting (which will be lower than the one you are using now). And if you notice that you made a mistake and the setting is too high or too low, you can of course regulate during cooking, but remember that the pan will react very sluggishly. So after each change, wait for several minutes until it takes effect, before intervening again.

All that time, you have to be aware that the appearance of the crust has nothing to do with the actual doneness. Either learn to recognize the doneness by touch/pressing, or, much simpler, use a meat thermometer.

Last but not least, consider also using methods beside simple "throw it on the pan and wait", for example Alton Brown's oven+pan steak. They are certainly more work, but they have a much higher chance of success for somebody who is starting out.

There is an extremely easy solution to this problem if you want to invest a little bit of money: sous vide. Cook to just under the desired temp (or just follow the guide for the cut on serious eats or other good cooking site) and sear the heck out of it on the cast iron like you did using a high smoke point oil (peanut or similar). It's cooking for dummies, basically impossible to mess up and should come out perfectly every time.

Heat moves slowly, and takes a while to travel into the middle your food. If your pan is very hot, the surface of your food gets heated so quickly that it burns before enough heat has got into the middle of your food.

The skill in cooking on a pan is finding the right combination of temperature and time, where the middle has time to heat up to a desired doneness, while the outside does not cook too far.

It's a skill you have to learn, because the right heat and the right time depend on what you are cooking (e.g. fish and meat cook differently), what its original temperature is (room temp or fridge) and most importantly how thick the piece is.

Searing is not cooking. You will need to finish the meat in a pre-heated oven. This is how it's done at restaurants, and seems to be what you are trying to replicate.

Searing will develop a delicious "crust" on the exterior of the meat as part of the maillard reaction, without the heat penetrating deep into the meat (which is what "cooking" is). To penetrate the heat, it takes time. To preserve the nice crust developed during a sear, it's best to finish cooking in an oven.

Steps:

1. Use a high-temp oil, such as Canola or Vegetable Oil. Olive oil will burn and leave a foul taste on the meat.

2. Preheat your oven (temperature depends on what meat you are cooking. For steak, 500? or its highest setting. This will differ slightly depending on the meat).

3. Place your cast iron pan on your stove on its highest temperature setting, with oil inside, and let is heat up for 5–10 minutes.

4. Place meat in pan, and allow to sear for 30 seconds up to 1 minute. Flip and repeat. Exact time will again, depend on the meat, its thickness and type.

5. After seared on both sides, remove the pan from the stove and place it into the pre-heated oven. Allow to cook for 2 minutes on each side (again, exact times depend on the meat, etc).

6. Remove from oven, transfer meat to a plate and loosely cover with aluminum foil.

7. Allow meat to rest for 5–10 minutes before cutting into it. This allows the juices inside the meat to settle and be "sucked" back into the meat while cooling, resulting in juicier meat. Cutting into the meat before allowing it to rest will result in the hot juices running out and leaving your meat dry.

"Anyone has any idea what I did wrong?

Actually, in one way you were highly successful. This is exactly what you want when cooking with cast iron. Ever have seared tuna at a nice restaurant? It's cooked on the outside and raw in the middle. If you attempt to cook the food through at searing temperature you will get something with the consistency of a hockey puck or it will become charcoal.

You seared perfectly. You just need to finish the food. What I do with my iron pan is sear, flip and then after a short time, cut the heat way down and cover to allow it to cook through. I have a universal lid that's reflective and has a vent to prevent steaming the food.

Use a thermometer and check the middle after some time has passed. It will often take a lot longer to cook through than to sear. You'll start to get a feel for how long you need for different items with practice.