Preheat oven to 400 degrees F and line a baking sheet with parchment paper. 

Mix hash browns, parmesan, and egg whites in a large bowl. If your hash browns are frozen, you can microwave them for 30-60 seconds on 50% power to soften them. 

Spoon mixture into 6 small circles on the baking sheet, spreading them as thinly as possible. Each "shell" should be slightly bigger than a street taco tortilla. 

Bake for 20-22 minutes, flipping halfway through. You can extend cook time if you'd like extra crispy shells, just be sure to keep an eye on them to avoid burning. Parmesan tends to go from yummy to charred in the blink of an eye.

After the shells have cooled, top with eggs, crispy carnitas, or any other toppings you'd like.