To make the pea pesto: Pulse together the peas, garlic, Parmesan cheese and salt and pepper in a food processor.
With the machine running, slowly add in the olive oil until it is well combined, which takes about one to two minutes.
Season with salt and pepper to taste.
Transfer to a small bowl and set aside.
To make 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, which takes about one to two 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 Fahrenheit until slightly crisp, which takes about five minutes.