Getting rid of raw onion taste without the frying process?

I used to cook in a school kitchen that had a lot of problems so I had to figure out untraditional ways of doing things. About once a week I would chop 4 full size pans worth of onions and bake them on 400. Took quite a long time but you can walk away and check and stir them once an hour after 2 hours. I always mixed in a little oil but I don't know how necessary that was.

Cook's Illustrated recently published a recipe for caramelized onions that took only 20 minutes. The key points:

• Cook them in some rapidly boiling water to break down the cell walls, about 15 minutes.
• After they are fully cooked, add a small amount of baking soda (about 1/4 tsp to three pounds of onions).

The slightly basic solution aids the quick conversion of starches to sugars, and aids browning. In five minutes, you cook off the excess water, as well as rapidly browning the onions without turning them crispy.

The small amount of baking soda suffices to change the reaction rate, without substantially altering the taste. The result is a big pile of sweet, brown caramelized onions; they can be turned into French Onion Soup simply by adding some stock.

Cook's Illustrated recommends slicing pole-to-pole (Lyonnaise) rather than across for this preparation. Apparently it makes them firmer; otherwise, they risk collapsing entirely.

Give this experiment a try, and you may be able to adapt the technique. I don't know if it would adapt well to smaller quantities, but you could cook three pounds and set the rest aside for many, many other uses. (Top goat cheese; put it on a pizza; add to green beans; save for another curry.)

@Spagirl's comment is right - cook too cool and they seem raw for ages. When I slow cook, to get nice soft onions, I lightly (or more, depending on colour) fry them first. In a heavy pan (not non-stick) you need very little oil to a lot of onions, so the fat content per portion is pretty much rounding error unless you carefully measure the oil you use later and always use the same meat (if any).

I've made the "basic curry sauce" from The Curry Secret successfully. It's reproduced here, but essentially par-cooks the onions, purees them in a blender with a little water and the other ingredients before boiling gently for a bit longer. It does use oil but on re-reading I don't think you'd need to use anything like as much, and you should lightly fry the spices in it. I've also found it to freeze well. You can soften/sweeten onions in a microwave, which might work in place of the first simmering step.

I haven't tried it in the slow cooker, but on high, starting with a boiling mix, it might just work. I had a slow cooker that was a little too hot on high and also had a metal inner pan that could be used on the stove. That would be good here, but I've gone back to the classic pottery type.

Bake, don't boil! Here's why:

What we call "caramelisation" in onions is really a combination of lots of processes, including enzyme reactions that produce simpler sugars, physical breakdown of the cells that release sweeter juices, dehydration which concentrates the flavours, plus the actual Maillard and Caramelisation reactions that give the distinctive brown colour and caramel notes.

Both Maillard and Caramelisation only really happen above around 150°C, so they're basically impossible to achieve while cooking in water. Cooking in water also dissolves the sugars, so they end up in the boiling water, rather than staying to sweeten the onions.

Caramelizing onions in the oven is very effective and really easy. Here's how I do it:

1. Slice a roasting dish full of onions, make sure the layer is at least 5cm deep so the lower layers can sweat and soften. You can chop and add your other veggies here, too.
2. Sprinkle and toss with salt. I sometimes add a small amount of olive oil, brown sugar/and or a splash of red or white wine vinegar at this point, though that might not be the flavour profile you want for curry bases. A bit of neutral flavoured oil won't hurt.
3. Add half a cup of water, and cover the pan tightly with foil.
4. Starting in a cold oven, roast at 150°C for 2 hours or so
5. Remove the foil, increase oven temp to 180°C, stir and continue to roast uncovered until you have the colour you want, maybe another hour.

At the end of this, you'll have a very soft, sweet 'onion jam', where the onion slices are barely holding together. Blending that mix with a bit of water should make a very nice sauce base.

I am more used to the term sweat onion.

I fry sweat onions with no oil on low heat. I just use a little orange marinade. On low heat you could even use nothing but onion and cover (open to stir). Once stared the onions release moisture. You can then remove the cover if you want to steam off water.

For the temperature to caramelize you need oil. You can push it to caramelize without oil but sticking can be a problem.

When making broth I usually roast an onion. So just stick it on and put over open flame until it's start to turn brown, 5 minutes maximum. If the edge start getting charred it's last call.
Then I just put with other vegetables and that's all.

And of course later I'm using veggies from that broth with a little chicken fat to make curry. I make it to taste like Japanese curry so probably something similar to British one (not so spicy and little bland compared to Indian one).

You can use a sous vide, three pounds at 85 - 90 degrees celsius for approximately 24 hours works. I've experimented throughout that range of temperature, with various quantities of sugar to aid caramelization and even tried starting the onions off in a very quick sauté prior to bagging and immersing them.

The best method I found was blanching the sliced onions in boiling water for a minute, then running them under cold water, bagging them and then forgetting about them in the circulator at 85 degrees for a day. Nothing else; no sugar, no baking soda, etc.

Note, they do not taste as good as what you'd achieve in a pan; I used this method when I needed a large quantity of them to puree for a sauce.

One of the home setups (I believe several are pretty popular on sites like Amazon right now) should work, my tests were done using a full-size hotel pan and a commercial circulator, so you might need to tweak a bit, but the temperature is what matters most. The sauce was a rich onion gravy with the consistency of something like demi glace, so it should lend well to being the base of a gravy.