Preheat oven to 400ºF.
Slice the tomatoes in half across the middle, making 2 tomato cups per tomato. Scoop out the tomato guts and discard.
Place a small skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Sauté the onion and garlic until soft, 3-5 minutes Transfer to a medium-size mixing bowl and let cool.
Add the mozzarella, prosciutto, breadcrumbs, basil and parsley to the bowl, season with salt and pepper, and stir to combine. Divide the mixture among the tomato cups. Transfer the tomatoes to a baking sheet, transfer to the oven and bake until the tops are golden brown, about 15-20 minutes.