Roasted Red Onions Stuffed With Thyme-Mascarpone Mousse
Preheat the oven to 400 degrees F. Line a baking sheet with aluminum foil and set aside.
Peel the onions. Trim the stem ends so they will stand upright, and cut 1/2-inch from the top. Rub 11/4 teaspoons of the oil on each onion and season with 1/8 teaspoon of the salt and a pinch of the pepper. Place on the baking sheet and roast until tender and caramelized around the edges, about 45 minutes. Remove from the oven and let cool.
When cool enough to handle, push out the centers, leaving a shell. Return one piece of the removed onion to each to form a bottom. Chop the remaining inner pieces.
In a bowl, combine the chopped onions, mascarpone, thyme, garlic, shallots, remaining 1/2 teaspoon salt, and 1/4 teaspoon pepper, and mix until blended. Stuff each onion with the mixture and return to the baking sheet. Bake until warmed through and the cheese melts, 5 to 6 minutes.
Remove from the oven and transfer to serving plates. Place 1 thyme sprig in the center of each onion, and serve immediately.