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. Lightly toast bread slices, then remove crusts and cut each slice in half or thirds. 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.