Preheat the oven to 425 degrees F. Spray the cups of 12-cup muffin tin with non-stick cooking spray.
Place 2 tablespoons of hash browns into each muffin tin cup. Spray the potatoes generously with more cooking spray. (This will help them crisp up a bit.)
Bake in the oven for 10 minutes. Remove from the oven. Using a cup that fits into the muffin tin, press the hash browns down so that they flatten and spread up the sides of the cup a bit.
Place back in the oven and cook for another 10 minutes.
Add the eggs to a medium mixing bowl and beat with a whisk until they start to turn fluffy. Add the salt and pepper and whisk to combine.
Remove the hash browns from the oven. Top each cup with a layer of diced onion, a layer of sausage, and then a layer of cheese. Pour the eggs over each cup to cover the ingredients.
Bake for another 15 min or until the eggs are set.
Remove from oven and allow to cool for 1-2 minutes in the muffin tin.
Run a knife around the edge of each cup and then scoop out the hash brown cups. Serve warm or at room temperature.