Preheat oven to 400°F (200°C) and generously spray a muffin tin with cooking spray.
In a bowl, combine the 3 cups of hash browns and 1 cup shredded cheese.
Pat the hashbrown mixture into the muffin pan covering the bottom and sides of each cup. Bake for 15 to 20 minutes until the hashbrowns are crispy. Remove the pan from the oven and reduce the temperature to 350°F (177°C).
While the hashbrowns are baking, beat the 8 large eggs, ½ cup milk, ½ teaspoon ground mustard, and ¼ teaspoon black pepper in a large measuring cup until combined.
Once the hashbrowns have finished baking, divide the 1 cup ham and 2 leeks evenly between the cups.
Fill each cup ¾ of the way full with the egg mixture.
Bake for 15 to 20 minutes until the eggs have set.
Notes
Don't skimp on the non-stick spray. Nothing is worse than leaving half of your crispy tater tots stuck in the muffin tin.The egg cups will puff up while they bake. Only fill the muffin pan ¾ of the way up to prevent overflow.These breakfast cups are easier to remove from the pan when they are hot. If they start to stick run a plastic knife around the edges to help loosen them.This flavor combination is just one iteration. Feel free to swap in your favorite fillings and make this dish your own.Storage: Store these breakfast cups in an airtight container in the refrigerator for up to five days.To reheat: Reheat the breakfast cups on a parchment-lined baking sheet at 350 degrees Fahrenheit for 15 minutes.Or place on a microwave-safe plate and microwave for 30 seconds. Be careful to not microwave these too long because they will become tough and rubbery.