Add olive oil to a cast iron skillet on medium-high heat. Once hot, add diced onion. Use a spatula to stir as it cooks. Sauté until close to tender. Then add minced garlic to skillet and sauté until golden, about one minute.
Meanwhile, whisk together the eggs, milk, salt, oregano, basil, cayenne pepper, ground mustard, and black pepper.
When the onion and garlic are done, turn off the stove. Spread across the bottom of the skillet so it is evenly distributed. Dice the jarred roasted red peppers. Add the red pepper to the skillet and spread as well. Layer the mozzarella cheese, parmesan cheese, and chopped fresh parsley on top. Pour your egg mixture over everything.
Bake until the center is just set, 16 to 20 minutes. Check to make sure the center isn't very wiggly and then remove from the oven. Let it rest for a few minutes before serving. The frittata may bubble a lot in the oven but will settle down as it rests.