Cook egg noodles according to package directions for 8 mins. Drain. Return noodles back to saucepan. Add 1/2 sick margarine and stir. Set aside.
In in microwave safe bowl add 1 stick of margarine and the sliced onions. Microwave on 60% power for 3-4 mins until onions are translucent. Add to noodles, stir. Pour noodles and onion mixture in the bottom of a large glass or metal 9x13" cake pan.
Make 6-8 cups of instant mashed potatoes according to box directions. Then mix in 1 cup of shredded cheddar cheese into prepared instant potatoes. Immediately poor potatoe/cheese mixture over the noodle/onion layer. Smooth out to cover all noodles.
Place yellow American cheese slices 3 rows across and 5 rows down. You should have enough cheese cover the whole top.
Preheat oven to 350 degrees and bake till warmed through and cheese is melted. 20-30 mins Enjoy!