Preheat the oven to 375ºF. Grease a large baking dish (about 9 x 12") and set aside
In a large skillet over medium heat, sauté the bell pepper, mushrooms, onion, and minced garlic for 5 minutes to soften. Stir in the olives and chopped pepperoni. Scoop the mixture into the greased baking dish, spreading evenly along the bottom.
In a large bowl, whisk the eggs, milk, and spices until combined. Pour the egg mixture over the meat and vegetables. Transfer the baking dish to the preheated oven, and bake for 25 to 30 minutes or until the eggs are set.
Meanwhile, make your pizza sauce by combining the tomato sauce, basil, garlic, oregano, and red pepper flakes in a small bowl. Season with salt & pepper to taste. Heat it over the stovetop on low or in the microwave, until warm.
Slice the frittata into 8 pieces & serve topped with pizza sauce.
Notes
If you're making this on a Whole30 and cannot find compliant pepperoni, feel free to substitute fully cooked compliant sausage or prosciutto. Ground beef would be tasty too!