1) For the pea pesto: Pulse together the peas, garlic, Parmesan, one teaspoon of salt and quarter teaspoon of pepper in a food processor. With the machine running, slowly add the olive oil until well combined, about one to two minutes. Season with additional salt and pepper, if needed. Transfer to a small bowl and set aside.
2) For the crostini: Preheat a stovetop griddle on medium-high heat. Brush both sides of the sliced bread with olive oil and cook until golden, about 1 to 2 mins. Transfer the bread to a clean surface and spread one to two tablespoons of the prepared pesto on each slice.
3) 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 150°C/ gas mark 2 oven until slightly crisp, about five mins.