Heat the oven to 350°F. Grease a 9x13 baking dish with nonstick cooking spray and set aside.
In a large skillet, brown the sausage over medium heat until fully cooked and no longer pink. Place sausage on a plate that has been lined with paper towels to drain the grease. Set aside.
Add the onion and red pepper to the skillet and cook over medium heat until tender. Add the garlic and cook for 2 minutes. Set aside.
In a large bowl, beat the eggs and whisk in the milk. Stir in the cooked vegetables, potatoes, spinach, 1 cup of the shredded cheese, and the sausage. Season with salt and pepper.
Pour the mixture into the prepared baking dish and top remaining cheese and green onions. Bake for 40-45 minutes or until the eggs are firm and the top is slightly golden brown. Let stand for 10 minutes before cutting. Cut into squares and serve warm.
Note: This casserole can be made in advance. Pour the mixture in the pan and refrigerate for up to 24 hours. Bake when ready to eat. You can also reheat the casserole.