Add the peas, Parmesan, garlic, fresh mint, and salt and pepper in a food processor and pulse to combine. With the machine running, slowly add the olive oil until well combined, about 1 to 2 minutes. Transfer to a small bowl and set aside.
For the crostini:
Preheat a stovetop griddle or grill pan on medium-high heat. Brush both sides of the bread with olive oil and grill until golden, about 1 to 2 minutes.
Transfer the bread to a clean surface and spread 1 to 2 tablespoons of the prepared pesto on each slice. Top with 2 tomato halves and transfer to a serving platter before serving.
*Cooks Note: If you don't have day-old bread on hand, you can dry out fresh bread by putting the slices in a preheated 300 degree F oven until slightly crisp, about 5 minutes.