If baking immediately, preheat the oven to 375 degrees F.
Spray a 9x13" baking dish with cooking spray.
Spread the frozen tater tots on the bottom of the baking dish.
Top with the diced ham and shredded cheese.
In a medium bowl, whisk together the eggs, milk, sour cream, salt, pepper, mustard, and garlic powder until well combined.
Pour the egg mixture over the tater tot/ham/cheese mixture.
If prepping for ahead of time, cover with foil and refrigerate until ready to bake, up to overnight.
If baking right away, bake in the preheated oven for 45-50 minutes, until the cheese is golden brown and the egg casserole is baked through.
If prepped ahead of time, take the egg casserole out and let sit at room temperature for 20-30 minutes prior to baking.
Preheat the oven to 375 degrees F.
Bake in the preheated oven to 50-60 minutes, until cheese is golden brown and egg casserole is baked through.
Top with chopped parsley or green onions and serve. Enjoy!