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. Remove from heat.
Pre heat oven to 350F / 180C. Lightly butter a 10 inch / 25 cm rectangle baking dish (approximately).
Place drained potatoes in a large bowl and mash with a potato masher, then add 3 tablespoons of butter, salt, pepper and milk, beat until creamy.(If needed add a little more butter and or milk). Taste for salt. 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!
Notes
The best potatoes to use for mashed potatoes are Russet potatoes or Idaho potatoes because of their high starch content. Yukon gold potatoes are another good option, the texture of Yukon gold is a bit more buttery and not quite as starchy.It's not necessary to peel the potatoes before boiling, just remove the skin before mashing. Store any leftovers in an airtight container in the fridge, make sure they have cooled completely, this will prevent sogginess. It will last up to 3-4 days in the refrigerator. Re heat in the oven, stove top or microwave, stir the leftovers to make sure they heat through.Cheesy mashed potatoes can also be frozen, freeze the completely cooled dish in a freezer safe container. They will last for up to two months in the freezer.