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These roasted onions turn jammy and sweet in the oven with olive oil and balsamic vinegar. Here’s the temperature and timing for this tasty side dish!

Want an impressive healthy side dish? Try these roasted onions! When I’m thinking of side dishes, I most often think of something like roasted broccoli or roasted potatoes. But roasting onions in a hot oven mellows their flavor until its deeply caramelized and sweet.

Pair that with a hint of balsamic vinegar and fresh thyme and those simple flavors make an effortless side dish that goes with almost anything. Here are a few tricks for my favorite way to make roasted onions!

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What You’ll Need For Roasted Onions

The great thing about roasted onions is: all you need is time. (And thyme. Sorry, bad joke!) But really: there’s nothing hard about this recipe, and the ingredient list is very short. Here’s what you need for roasted onions:

Onions: You can use any variety! I like to use 2 red onions and 1 white onion. Each type of onion has a slightly different flavor. The hot oven transforms onions into a very mild flavor. Red onion has a beautiful purple color, so it makes for a stunning look.
Olive oil: My go-to oil for cooking.
Balsamic vinegar: This adds a pop of acidity to the flavors.
Fresh thyme: You can use other fresh herbs too, like chives or a hint of rosemary.

Tips For Cutting The Onions

To make the very best roasted onions, it starts with your knife skills! The best way to slice onions for roasting is not into rings: it’s into elegant slivers. This type of cut is often called French cut, though it doesn’t really have a standardized name. Here’s what to do:

Slice off the ends, then cut in half: Once the ends are removed, stand the onion on its flat cut side and cut it down the center.
Remove the paper. With your fingers, remove the paper and outer layer of the onion.
Cut following the curve of the onion. Place the onion on its flat side then cut slices following the curve of the onion, from the tip to the root. Want to see? Go to minute 1:40 of this How to Cut an Onion video.

How to Roast Onions in the Oven

You barely need a recipe for roasted onions! Once you’ve made it once, you can memorize it and repeat. All you need to remember is the oven temperature and how long to roast the onions. Interestingly, you can roast onions along with another vegetable at very high heat. But on their own, you’ll need to reduce the oven temperature! Jump to the full recipe or review the following:

Roast onions at 375°F for 45 to 55 minutes. The timing will depend on the age of the onions and how thick you cut the slices.
Stir at 30 minutes. Give the onions a stir at 30 minutes, or they’ll get a little too blackened. Then you can add the fresh thyme.
How to season roasted onions? Drizzle on 2 tablespoons olive oil and 1 tablespoon balsamic vinegar for every 3 onions.

Flavor Variations to Try

Fresh thyme and balsamic are what we think is the best way to season roasted onions. But there are lots of ways to mix up the seasonings. Try the following:

Dijon mustard: Add a dollop of Dijon mustard with the olive oil
Fresh herbs: Use fresh rosemary or oregano instead of thyme
Honey or maple syrup: Add a hint more sweetness with a touch of honey or maple

What to Serve With Roasted Onions

Roasted onions are a simple and elegant side dish to all sorts of main dish options. Here are a few ideas for what to serve with them:

Fish: Try Quick Broiled Salmon, Easy Pan Seared Salmon, Pesto Salmon, or Pan Fried Cod
Shrimp or Scallops: Opt for Creamy Shrimp Pasta, Easy Pesto Shrimp or Pan Seared Scallops
Chicken: Poultry goes especially well with roasted onions! They also work well as a Thanksgiving side dish or Christmas side.
Vegetarian/vegan main dishes: Try Eggplant Parmesan or Perfect Zucchini Lasagna

Storage and Leftovers Notes

Leftovers store refrigerated for up to 4 days. They’re fun to repurpose in sandwiches like a Pesto Sandwich or a Gourmet Grilled Cheese, or as a pizza topping (see my Best Pizza Recipes).

More Onion Recipes To Try

Pickled: Pickled onions add a briny tang to tacos, salads, and more! Try these 1-Hour Quick Pickled Onions.
Caramelized: Cook them 1 hour until they’re sweet and tender with Perfect Caramelized Onions.
Sauteed: In just 10 minutes, Simple Sauteed Onions are browned and tender.
Grilled: Try these smoky Grilled Onions, a fantastic summer side dish.

Dietary Notes

This roasted onions recipe is vegetarian, vegan, plant-based, dairy-free and gluten-free