The primary ingredient in pesto are the leaves of the basil plant. Basil leaves are the primary ingredient in pesto. Pesto is a versatile sauce that can be used to make soup. Garlic is often used in making pesto. Olive oil is typically used to make pesto.Chopped tomatoes can be added to basil and olive oil in a variation on traditional pesto.Red bell peppers can be used as a base to make a red pesto.he primary ingredient in pesto are the leaves of the basil plant. Basil leaves are the primary ingredient in pesto. Pesto is a versatile sauce that can be used to make soup. Garlic is often used in making pesto. Olive oil is typically used to make pesto.