Roasting Onions In Oven. Toss a big pile of thinly sliced onions with olive oil and a splash of water (which will. To caramelize onions in the oven, heat the oven to 400° and line a rimmed baking sheet with parchment paper.

Roast the onions until the bottoms are golden, about 20 minutes. Place onions on a baking pan. Set a sprig of rosemary in with the onions, tossing it with the oil and onions and leaving it whole to scent the onions as they roast.toss a bit when you take the onions out of the oven, so a few of the roasted rosemary leaves fall into the onions.toss in whole cloves of garlic along with the onions for an.