Prepare the sausage: Cook the sausage in a skillet over medium heat until browned and fully cooked. Drain any excess grease.
Mix the base: In a large mixing bowl, whisk together eggs, milk, garlic powder, thyme, salt, and pepper.
Assemble the casserole: In a greased 9x13 baking dish, layer the bread cubes, cooked sausage, diced bell peppers, spinach, and onions. Pour the egg mixture evenly over the top. Sprinkle shredded cheese as the final layer.
Rest: Allow the casserole to sit for 10-15 minutes so the bread can soak up the egg mixture.
Bake: Preheat the oven to 375°F (190°C). Bake the casserole uncovered for 40 minutes or until the center is set and the top is golden brown.
Serve: Let the casserole cool for 5 minutes before slicing and serving.
Notes
Make it the night before for a stress-free morning. Simply assemble, cover, and refrigerate overnight, then bake fresh on Christmas morning.