Arrange peppers, open sides up, on a large baking sheet.
Top half of each pepper with about 2 teaspoons pesto (or 1 basil leaf), 1 slice mozzarella, and some prosciutto and fold other half of pepper over prosciutto to enclose filling.
Drizzle peppers with oil and bake in middle of oven 10 minutes, or until cheese is melted. Serve!