Boil 6 cups of water over high heat. Add pasta and peas, turn heat down to medium-high and stir occasionally to prevent sticking until el dente (approximately 8 minutes).
Meanwhile, preheat oven to 375F.
Open the cheddar cheese soup and mayonnaise, and combine in a large bowl. (Use milk as a substitute for the mayonnaise if you'd like a lighter meal.)
Drain the pasta and peas, and then toss in the soup-mayonnaise sauce.
Pour the contents of the bowl into a casserole dish. Top with breadcrumbs or flaked breakfast cereal if you'd like a crunchy topping.
Bake for 20-40 minutes until the casserole reaches the texture you'd like. (If you'd like soft and cheesy, cook for less time; cook longer if you'd like a crunchy pasta topping and the texture to resemble what I've shown here.)