Pesto: Pulse together the peas, garlic, Parmesan and salt and pepper in a food processor. With the machine running, slowly add the olive oil until well combined, about 1 to 2 minutes. Season with salt and pepper, to taste. Transfer the pesto to a small bowl and set aside.
Crostini: Preheat a stovetop griddle or grill pan on medium-high heat.
Brush both sides of the sliced bread with olive oil and griddle 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 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 the oven at 300 degrees F until slightly crisp, about 5 minutes.