Caramelizing onions always takes longer than you think it is going to. This is partially the fault of recipe writers who tell you it’s something you can do in half an hour, which is just not true. Sure, you can brown onions in half an hour, but caramelization — the process of breaking down long chains of sugar molecules into single sugar molecules, then further breaking those sugars down into smaller, flavorful molecules — takes at least 40 minutes for a single onion.
My favourite way to caramelize onions is in a pan set over low heat. There are ways to speed it up, like adding extra sugar and baking soda, but my laziness trumps my impatience every time, so I let the naturally occurring sugar in the onions take their time.
But a sauté pan can only hold so many onions. If I need two or more onion’s worth of jammy, deeply flavored alliums — or have simply run out of stove space — I’ll caramelize them in the oven, which takes a while but is fairly hands off, and doesn’t require as watchful of an eye.
You gotta keep ‘em wet
When I first started caramelizing onions this way, I found a lot of recipes that suggested adding a “splash” of water to keep the onions from scorching. This was not enough. I start by adding a whole cup of water to the pan, and continue adding cups of water to the pan as they cook. This keeps the environment from being too drying, and keeps the onions from browning before they caramelize, and is especially important if you’re using your oven’s convection setting.
Other than onions and water, all you’ll need is a little salt and some olive oil. Then, it’s simply a matter of waiting and stirring every 15 minutes or so, until they are reduced to a deeply coloured, soft and supple mass of onions, perfect for adding to a dip, soup, or anything else that could use a scoop of sweet, sweet onions.
Keep ‘em together
When roasting vegetables, you want to spread the plant parts out, to keep them from getting soggy and allow the edges to brown and crisp. The latter is the opposite of what we’re going for here, and the former is the opposite of what we’re going to do. Instead of spreading our onions out on the pan, we’re going to keep them lumped together. This keeps everyone moist as they caramelize, and ensures the end pieces don’t cook much more quickly than the onions in the centre.
In addition to adding water every 15 minutes (or as it evaporates), you’ll want to stir just as frequently. Do this by scooping the onions off the sheet pan and inverting them, flipping towards the centre of the pan so the outside becomes the inside and the top becomes the bottom. Toward the end, as the onions get darker and darker, you’ll want to do this every 10 minutes or so, then every five, until you reach your desired level of caramelization.
Oven Caramelised Onions
- 2 large onions, halved and sliced thinly
- 2 tablespoon olive oil
- 2 teaspoons salt
Line a sheet pan with a silicone baking mat, or sheet of parchment paper, and heat your oven to 180°C. Add the onions to the pan, and toss with oil and salt. (Use your hands.)
Gather the onions into a pile in the middle of the pan, then add 1 cup of water and place on a rack set in the centre of your oven. Cook for 15 minutes, then stir the onions by scooping the onions off the sheet pan and inverting them, flipping towards the centre of the pan so the outside becomes the inside and the top becomes the bottom. Repeat every 15 minutes until the onions start to darken and caramelize.
You will notice a few browned, or even burnt bits here and there, and that’s alright. If you notice this happening a lot, you can stir more frequently, add a little more water, and pat down the onions after stirring to ensure you don’t have any bits sticking up.
Once the onions start to look golden brown, begin stirring every 5-10 minutes, until you reach your desired level of caramelization. For two large onions, this takes a couple of hours. Mix into dips, spread into grilled cheese sandwiches, or stir into soups or casseroles. You can also just eat them with a fork. There’s nothing quite like them.
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