Preheat Oven: Set to 350°F and grease a 9×13 baking dish.
Cook Noodles: Boil egg noodles, undercooking them by 2 minutes. Drain well.
Sauté Onions: Melt butter in a skillet over medium heat, cook onions until soft and translucent.
Prepare Mixture: In a large bowl, mix cream of mushroom soup, mayonnaise, sour cream, milk, salt, pepper, and sautéed onions with butter. Stir in tuna, peas, and cheddar cheese.
Combine & Assemble: Gently fold in the noodles until evenly coated. Transfer mixture to the baking dish and sprinkle crushed potato chips evenly on top.
Bake: Place uncovered in the oven and bake for 30 minutes, until hot and bubbly.