Slow Roasted Red Onions

Roasted Red Onions are an unexpected and amazing vegetarian side dish -- or an ingredient that can be added to dozens and dozens of recipes. The slow roasted whole onions transform from raw, firm and sharp, to sweet, soft, and aromatic.

It's always fun to present something that might be new to friends and family. Especially something like taking one simple ingredient and turning it into an amazingly special dish.

I cook onions nearly every day, though they are typically the foundation for a dish. However, these Roasted Red Onions are the dish. Whole roasted onions are a deeply flavored, subtly sweet vegetarian side dish.

Jump to:

• The Ingredients
• Substitutions
• How to Make Them
• Recipe Tips
• Serving Suggestions
• Other Delicious Onion Recipes
• Making Them Ahead and Storage
• What happens to onions whey they're slow roasted?
• Roasted Red Onions Recipe

The Ingredients

This recipe is really all about one everyday, simple ingredient. A red onion.

(I often suggest brands I love and use -- these are only suggestions, and this is not a sponsored post.)

• red onions - Choose onions that are heavy for their size and without bruises. For this recipe, we want very small red onions, about 5 to 6 ounces each.
• extra virgin olive oil - I use this one.
• fresh herbs (rosemary, thyme, oregano) - Look for perky bunches and avoid any brown, bruised or yellow leaves.
• butter - I always cook (and bake) with unsalted butter so I can control exactly how much salt is in the recipe.
• salt - I cook with Kosher salt. I prefer for a few reasons: its larger flake size, it's less refined and usually doesn't contain additives, and it does a great job enhancing the flavor of foods without making them taste salty.
• black pepper - Preferably freshly ground.

(See recipe card below for quantities.)

Substitutions

• You can use yellow or white onions for this recipe as substitutions for the red, if you prefer. The key is that they're not too big -- 5 to 6 ounces is ideal.
• I love the combination of rosemary, oregano and thyme with roasted red onions, but you can substitute, or use more or less of any of them. Marjarom or sage would both work well.
• If you want to keep the recipe vegan, substitute the butter with more of the olive oil.

How to Make Them

- Preheat the oven to 300°F, and adjust a rack to the center.

- Leaving the skin on, wash and dry the onions.

- With the skin still on, cut the root end off of each one, just about ½-inch, only enough to create a base. Then, slice about 1-inch off the top of each one, and use the knife to mark an “X” the surface, only cutting into it about ½-inch or so.

- Place them in a 9 x 13 x 2-inch pan, on their bases. Drizzle about 3 tablespoons of the olive oil on the flat surface of the onions -- over the "X." Sprinkle generously with salt and pepper and place them in the preheated oven for 1 hour.

- Remove them from the oven and drizzle with the remaining 2 tablespoons of oil. Cover the pan lightly with foil, return them to the oven for 1 more hour.

- Remove the onions from the oven again and take off the foil. Add about half of the fresh herb sprigs to the pan, surrounding, and on top of the onions.

- Evenly distribute the butter among the onions, on the top center of each one. 
 
Uncovered, return the onions to oven and roast for another 30 minutes.

- Baste the onions with the juices from the bottom of the pan and turn the heat up to 350°F.

- Continue to roast for 1 to 2 more hours, basting them every 20 minutes with more juices from the bottom of the pan. (It's your call -- the longer you roast, the more caramelized and soft they'll be.)

And for an even deeper flavor . . . when you turn the oven up to 350°F, peel and cut the onions into quarters from the "X" at the top.

- Remove from the oven and let them cool in the pan. If you haven’t already done so, when they’re cool enough to touch, take off the skins and slice. Serve with the remaining fresh herbs.

(More detailed instructions are in the recipe card below.)

Recipe Tips

• I add fresh rosemary, oregano and thyme on their sprigs, breaking them into pieces if they're extra long. If the leaves of the herbs are especially big, they can be roughly chopped.
• There are set cooking times in the recipe card below, but you don't have to be exact with them. The key is knowing that the longer they onions cook, the sweeter and softer they'll be.
• If at any point while the onions are uncovered they seem like they are becoming too dark (darker than golden), lightly cover them with foil.
• Cooking times may vary, depending on the size of the onions -- just be sure they're very, very soft when they're done.
• And a final tip, don't wash the baking dish right away, and use it to roast other vegetables, like carrots or potatoes. They'll soak up the leftover juices from the onions and be rich and delicious. If you're not going to use it right away, just cover it with foil and refrigerate until you do.

Serving Suggestions

• These whole roasted onions can be served on their own as a side dish. They're excellent served alongside Lavender-Rosemary Grilled Ribeye Steak, Rosemary Lamb Skewers, and Blackberry Butter Roasted Chicken. (You can either slice the onions or serve them whole, for a more dramatic presentation.)
• They can also be added to stews, rice dishes and how about mixed into scrambled eggs? (This is excellent, trust me!)
• Roasted red onions can become so soft that you can practically spread them on bread. Imagine a layer beneath mashed avocado. It's makes for an incredible avocado toast!

Other Delicious Onion Recipes

• Grilled Green Onions
• Lime Pickled Shallots
• The Best Caramelized Onions
• Tomato Onion Salad
• Braised Pearl Onions and Chestnuts

Making Them Ahead and Storage

• The onions are at the best when eaten the day, or day after they're made.
• They can be stored for about a week in the refrigerator in an airtight container.
• To reheat them, place them in baking dish and in a 375°F oven until hot.
• I do not recommend freezing them.

What happens to onions whey they're slow roasted?

Much like when we slow roast garlic, roasting onions for a long time at a low temperature softens them and slowly draws out the natural sugars as they cook, caramelizing them, almost from the inside out.

With no sugar added, roasted red onions are unbelievably sweet, and while they still taste like onions, they're incredibly mild.

I hope you find these roasted red onions to be as incredible as I do!