Heat a skillet on medium heat and add the pork sausage. Cook the meat, stirring and breaking it up as it cooks, until it is fully brown and cooked through. Remove from heat and set aside to cool slightly while you prepare the other ingredients.
Preheat oven to 350F. Lightly spray the cups in a muffin pan and set aside.
Whisk the eggs and milk in a mixing bowl. Season with salt and pepper, to taste.
Stir in the chopped spinach, tomatoes, and shredded cheese. Add the cooked and cooled sausage and stir to distribute.
Divide the egg mixture evenly into the 12 cups in the prepared muffin pan.
Sprinkle additional shredded cheese into each cup, as desired.
Bake for 15-20 minutes at 350F.
Remove from oven and let cool slightly in the muffin tin, then serve.
