In a large skillet over medium heat, cook the sausage until browned. Remove sausage and set aside.

In the same skillet, add onions and bell peppers, cooking until they are softened, about 5 minutes. Add spinach and cook until wilted, about 2 minutes.

In a large bowl, whisk together eggs, milk, salt, and pepper. Stir in the cooked sausage, vegetables, and shredded cheese.

Spread the frozen diced potatoes evenly in the prepared baking dish. Pour the egg mixture over the potatoes. Sprinkle additional cheese on top if desired.

Bake in the preheated oven for 35-45 minutes, or until the eggs are set and the top is golden brown.

Let the casserole cool for 10 minutes before slicing and serving.