The leaves of the mustard plant are called mustard green. They are either flat or crumbled in texture. These leaves when added to any recipe lend it a pungent and peppery flavor. These leaves are most readily available from the month of December through April. Some recipes that use mustard green as an ingredient include Northern Italian Pasta Shell, Northern Italian Pasta Shell Stuffing, Pasta Shell Bolognese Sauce, Pasta Shell Broccoli, Pasta Shell Cheese, Pasta Shell Chicken, Pasta Shell Chicken Broccoli and Pasta Shell Corn Bacon.