Preheat oven to 350. Grease 9x13 glass baking dish.
in large stockpot, prepare mac'n'cheese as directed. Mix in the pot. Once finished, mix in peas, soup and tuna. Stir well.
Transfer macaroni mixture to prepared 9x13. Top with American cheese slices, evenly spaced. Bake for 5 minutes. Top with fried onions. Bake for 5 more minutes. Allow to rest for about 5 minutes, then slice and serve!