French Onion Casserole

Name: French Onion Casserole
Ingredients: Cooking spray; 3 tablespoons olive oil; 2 large sweet onions, thinly sliced (about 7 cups); 1.50 teaspoon kosher salt, divided; 2 teaspoon apple cider vinegar; 2.50 cups heavy cream; 2 cups unsalted vegetable stock; 1 16-oz. pkg. penne rigate or ziti pasta; 1.50 tablespoon Worcestershire sauce; ⅛ teaspoon ground nutmeg; 1 medium clove garlic, grated; 12 ounces aged Gruyère cheese, shredded (about 3 cups), divided; 4 ounces sharp white Cheddar cheese, shredded (about 1 cup), divided; 1 tablespoon fresh thyme leaves