Cheesy Kale Breakfast Casserole

Name: Cheesy Kale Breakfast Casserole
Ingredients: 2 tablespoons unsalted butter; 1 yellow onion, diced; 16 ounces crusty sourdough, cut into cubes; 1 cup shredded mozzarella (about 4 ounces); 1 cup shredded Gruyere or Swiss (about 4 ounces), plus more for topping; 6 ounces kale, chopped (chard or spinach would always work!); 3 1/2 cups whole milk; 1/2 cup heavy cream; 2 teaspoons Dijon mustard; 1 1/2 teaspoons kosher salt; 1/4 teaspoon grated nutmeg; 8 large eggs; 4 sprigs fresh thyme, leaves only; A few shakes hot sauce; Freshly ground black pepper