For the pea pesto: Pulse together the peas, garlic, Parmesan, 1 teaspoon of salt and 1/4 teaspoon of pepper in a food processor. With the machine running, slowly add the olive oil until well combined, about 1 to 2 minutes. Season with additional salt and pepper, if needed. 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 sliced 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 tomato halves and serve.
*Cook's 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.