Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish with butter or non-stick spray.
In a large skillet over medium heat, cook the sausage until browned and crumbled; remove with a slotted spoon and drain on paper towels.
Add olive oil to the same skillet and sauté the bell peppers and onions for 5 minutes until softened.
Stir in the chopped spinach and cook for 1 minute until wilted, then remove from heat.
In a large bowl, whisk together the eggs, milk, salt, black pepper, and garlic powder until well combined.
Arrange the bread cubes in the bottom of the prepared baking dish.
Top the bread evenly with the cooked sausage and the sautéed vegetable mixture.
Sprinkle the shredded cheddar cheese over the top of the sausage and vegetables.
Pour the egg mixture slowly over the entire dish, ensuring all bread pieces are submerged or coated.
Bake for 40 to 45 minutes until the center is set and the top is golden brown.
Let the casserole rest for 10 minutes before slicing and serving.