Heat a medium cast-iron or ovenproof skillet over medium heat. Once hot, add the pepperoni and cook, stirring, until crisp, 4 to 5 minutes. Add the onions, bell pepper, oregano, garlic, 3/4 teaspoon salt and a few grinds of pepper. Continue to cook, stirring, until the onions are very soft and start to turn brown, about 5 minutes.
Stir in the tomato paste and cook, stirring, until it coats the vegetables and smells lightly toasted, about 1 minute. Stir in the tomatoes. Reduce the heat and simmer, stirring occasionally, until the mixture is slightly thickened, about 15 minutes. Stir in the olives and Parmesan and remove from the heat. Season with additional salt and pepper.
Scatter the mozzarella over the dip in the skillet. Put the skillet under the broiler and broil until the cheese is melted and brown, 2 to 3 minutes. Top with the basil and crushed red pepper flakes. Serve warm with crusty bread for dipping.
Copyright 2016 Television Food Network, G.P. All rights reserved.