Croissant Breakfast Casserole

Name: Croissant Breakfast Casserole
Ingredients: 1 pound thick-cut bacon, cut into 1/2-inch strips; 2 tablespoons unsalted butter, plus more for the baking dish; 2 medium onions, very thinly sliced; Kosher salt; One 8-ounce package cream cheese; 2 tablespoons finely chopped fresh chives, plus more for garnish; 2 scallions, thinly sliced crosswise; 2 teaspoons fresh thyme leaves; Pinch freshly grated nutmeg; Pinch cayenne; 2 cups shredded Gruyère (about 8 ounces); 3 tablespoons Dijon mustard; 12 large eggs; 1 1/2 cups half-and-half; Freshly ground black pepper; 10 plain croissants, toasted; Nonstick cooking spray