In a large pot of boiling water cook potatoes until tender, drain well.
In a small frying pan add 1 tablespoon of butter and chopped onion sauté until slightly golden.
Pre heat oven to 350°. Lightly butter a 10 x 7 inch baking dish (approximately).
Place drained potatoes in a large bowl and mash with a potato masher, then add 3 tablespoons of butter and milk and beat until creamy.(If needed add a little more butter and or milk). Fold in onion butter mixture and shredded mozzarella cheese.
Spoon into prepared baking dish. Top with Parmesan and chopped parsley. Bake for approximately 10 - 15 minutes (I broiled it for the last 5 minutes to brown the top). Let sit a couple of minutes before serving. Enjoy!