Preheat your oven to 400º. While it is heating up, spray a 9x13 pan with cooking spray.
In the meantime, dice a half a sweet onion and lightly sauté on the stove until translucent and fragrant.
Pour the hashbrowns into a large bowl and stir in the sautéed onions and a sprinkle of salt and pepper. Toss to combine.
Layer the potato and onion mix into the prepared pan and spread evenly.
Melt a stick of butter and drizzle over the potatoes.
Bake the hashbrowns for 25-30 minutes or until they begin to brown. Remove from the oven and allow to cool.
While the hashbrowns are cooling, you can shred your cheese and dice your ham.
Once your hashbrowns have cooled, sprinkle on the cheeses evenly, and then the ham. Cover and refrigerate overnight.
Crack your eggs into a large bowl and add the evaporated milk, kosher salt, pepper, seasoned salt and dry mustard.
Beat with a hand mixer until well combined. Once combined, cover and refrigerate overnight.
Preheat your oven to 350º. Remove your hashbrown pan and egg bowl from the fridge and allow to warm up a bit.
Give the eggs another good whisking and then pour the mixture evenly over the hashbrowns.
Bake the casserole uncovered for 45-55 minutes or until the eggs are bubbly, lightly browned and set in the middle.
Slice and serve warm.
