Preheat the oven to 350°F (175°C). In a skillet, heat olive oil over medium heat. Add diced onion and bell pepper, sauté until softened, about 5 minutes.

Add chopped spinach to the skillet and cook until wilted, about 3 minutes. Remove from heat and set aside.

In a mixing bowl, whisk together eggs, milk, salt, black pepper, and garlic powder until well combined.

In a greased 9x13 inch baking dish, layer the cubed bread (if using), followed by the sautéed vegetables. Pour the egg mixture evenly over the top.

Sprinkle shredded cheese on top of the casserole.

Bake in the preheated oven for 30-35 minutes, or until the eggs are set and the top is golden brown.

Let it cool for a few minutes before slicing and serving.