Replacing quick-cooking oatmeal in a two-ingredient cookie recipe with cooked steel-cut oats - why doesn't it work?

If you want to use steel-cut oats/oatmeal, you'll probably want to start with a recipe that calls for it. You can substitute old fashioned oats instead of quick-cook oatmeal in most (possibly all) cookie recipes but you can't substitute cooked oatmeal without making major adjustments.

In this case, old fashioned oats are specifically called out in the recipe as a substitute with the note that quick cook oats will give the best result:

• Quick oats give the best results, however you CAN use rolled oats. Use Gluten Free Oats for gluten free cookies.

Oats are a dry ingredient. There's no water in them so they absorb moisture from the other ingredients to cook. In this case, the banana. When you cook the oats first, you're introducing a lot of extra water and it's likely to make them take much longer to cook and change the texture.

In a recipe for cookies made from instant steel cut oats, I found this substitution note:

If you’re using traditional steel cut oats (not quick cooking), you’ll want to cook them first, just like when you’re making oatmeal. To use cooked oats, simply substitute one cup of cooked oatmeal for the instant raw oats listed in the recipe, and add an extra half cup of flour (or more as needed to achieve a non-runny dough consistency).

You might be able to salvage this recipe by doing something similar but if you really want to use steel cut oats, you'll get better results by finding a recipe that actually calls for them. If you want to use this specific recipe, use old fashioned or quick-cooking oats.

Unlike Catija, I think your mistake is adding water - twice! The recipe you linked to does not ask for water in the mix, and it only asks for the oats.

Your first mistake is making "cooked oatmeal", by which I assume you mean porridge. So instead of finely-ground dry oats, you've got a soggy mush to start with. And then you add more water.

You even call the result "batter". You don't want anything that's even close to batter though - you want something that holds its shape like grainy modelling putty. As the recipe says, these cookies don't expand at all, because there's no baking soda or other raising agents - how they go on the baking sheet is the exact shape they come out. If you're pouring on a thin sheet of batter, it'll come out as a thin sheet of something very un-cookie-like.

I've done recipes like this with rolled oats. They'll come out more flaky than the original recipe, but they're perfectly tasty. But you do need to follow the recipe - when it says "two ingredients", it really means "two ingredients", and water is not one of them!

If you want a texture more like the original recipe, my top tip is to sieve the rolled oats through a colander with holes which are just big enough to hold back whole oat flakes. You'll end up with a colander full of whole flakes, and a bowl full of oat flour and partial flakes. The contents of the bowl will make this recipe perfectly. And the sieved oat flakes are perfect for making granola bars (what we call "flapjacks" in Britain), because you get a better texture on them by getting rid of the flour.