Heat oven to 400°F. Lightly grease 10-inch cast iron skillet.
Prepare the peppers and tomatoes, and chop the pepperoni.
Whisk together milk, melted butter, and eggs in large bowl. Add cornbread mix, pepperoni, peppers, tomatoes, oregano, and shredded cheese. Stir until blended.
Pour batter into prepared skillet.
Bake 24 - 26 minutes or until a toothpick inserted in center comes out clean. Top should be golden brown.
Serve warm with marinara sauce, if desired.