Macaroni And Cheese With Caramelized Onions

Preheat oven to 350°F with oven rack about 8 inches from heat source. Lightly coat a broiler-safe 13- x 9-inch baking dish with cooking spray; set aside. Cook bacon in a large Dutch oven over medium, stirring occasionally, until crisp, about 6 minutes. Using a slotted spoon, transfer bacon to a paper towel-lined plate, reserving drippings in Dutch oven.

Add butter to reserved bacon drippings, stirring over medium heat until melted. Add onions; cook, stirring occasionally, until onions are soft, 5 to 7 minutes. Reduce heat to medium-low; cook, stirring occasionally, until onions turn golden brown, 30 to 35 minutes (stir in a splash of water if onions begin to stick).

Add milk, salt, Dijon mustard, Worcestershire sauce, garlic powder, black pepper, and ground turmeric to onions in Dutch oven. Bring to a simmer over medium; cook, stirring occasionally, about 5 minutes. Whisk together cornstarch and cold water in a small bowl. Drizzle cornstarch mixture into onion mixture; cook, whisking constantly, until thickened, about 2 minutes. Remove from heat, and gradually add 1 1/4 cups each of the Cheddar and Gruyère cheeses, whisking constantly until melted and smooth, about 2 minutes. Stir in cooked pasta; pour mixture into prepared baking dish.

Bake in preheated oven until bubbly and golden, 20 to 25 minutes. Remove from oven; increase oven temperature to broil. Sprinkle evenly with remaining 1/4 cup each Cheddar and Gruyère cheeses; broil until cheeses are melted and golden, 3 to 4 minutes. Remove from oven; cool slightly on a wire rack, about 10 minutes. Sprinkle with chives and bacon just before serving.