Preheat oven to 350 degrees F. Spray a 9×13 baking dish with non-stick cooking spray. Prepare egg noodles, making sure to slightly under cook them by 2 minutes so they are slightly firm. Drain well.
In a skillet over medium to medium-high heat melt butter and add onions. Cook until onions are tender and translucent.
In a large bowl add cream of mushroom soup, mayonnaise, sour cream, milk, salt, pepper, and the cooked onions and butter from skillet. Mix well. Next, add tuna, peas, cheddar cheese and mix well. Last, add prepared egg noodles, gently folding into soup/tuna mixture until well blended. Pour mixture into casserole dish. Top evenly with crushed potato chips.
Place casserole on middle rack of oven and bake for approximately 30 minutes uncovered, until casserole is completely heated through. Enjoy!
