1/2 cup extra-virgin olive oil
1 small onion, chopped
2 cloves garlic, chopped
1 stalk celery, chopped
1 carrot, chopped
kosher salt, to taste
freshly ground black pepper, to taste
2 (28-ounce) cans crushed tomatoes
4 to 6 basil leaves
2 dried bay leaves
4 tablespoons unsalted butter, optional