French Onion Casserole

Preheat oven to 350°F with 1 rack in center of oven and 1 rack 6 inches from heat. Lightly coat a 13-by-9-inch broiler-safe baking dish with cooking spray.

Heat oil in a large skillet over medium-high. Add onions and 1/2 teaspoon salt; cook, stirring occasionally, until onions begin to soften, about 5 minutes. Reduce heat to medium; cook, stirring occasionally, until onions are deep golden brown and jammy, 15 to 18 minutes (add 1/4 cup water at a time if onions start to burn). Remove from heat and stir in vinegar.

Meanwhile, add cream and stock to a medium saucepan. Bring to a simmer over medium-high. Add pasta; simmer, stirring occasionally, until just al dente, 8 to 10 minutes. Stir in Worcestershire sauce, nutmeg, garlic, and remaining 1 teaspoon salt. Remove from heat. Add 2 1/2 cups Gruyère and 1/2 cup Cheddar, a handful at a time, stirring until melted between additions.

Stir half of caramelized onions into pasta mixture. Transfer to prepared baking dish and top with remaining caramelized onions. Sprinkle with remaining 1/2 cup each Gruyère and Cheddar. Bake on center rack until bubbly, about 20 minutes. Transfer baking dish to top rack and increase oven temperature to broil. Broil until cheese is golden brown, about 3 minutes. Remove from oven and sprinkle with thyme. Let cool for 10 minutes before serving.