For the noodles, you can do this several ways; 1. Use boil first noodles and cook until just before done and drain and use. 2. Use oven ready noodles but briefly boil them first to just soften them up and drain and use. 3. Make your own egg pasta noodles and precook for 30 seconds in boiling water and drain and use.
Preheat your oven to 190 C. Grease a 9x13 baking dish with butter. Prepare the noodles as directed above.
In a large skillet, heat the butter on medium heat and add the onion and season with salt and pepper to your liking. Cook until soft, 10-15 minutes.
Place 4 noodles in an even layer on the bottom of the baking dish. Layer 1/2 of the onion over the noodles then layer 1/2 of the mashed potatoes over the onion, then 1/2 cup of cheese.
Repeat with another layer of noodles, remaining onion, remaining potatoes, and 1/2 cup of cheese.
Place the last noodles on the top and the remaining cheese.
Bake for 30 minutes or until hot and bubbly. Serve hot.