Preheat oven to 325*F
In a large bowl, whisk together eggs, milk, salt, pepper, cayenne, cheese.
Preheat cast iron skillet to medium low.
Dice onion and add to skillet. Sauté for 3 to 5 minutes until softened.
While the onion cooks, dice yellow potatoes into small 1/4 inch pieces.
Add potatoes into skillet in a single layer. DO NOT TOUCH. Cook for 5 minutes. Flip and sauté another 5 minutes until cooked through.
Meanwhile, chop asparagus into small pieces (about 1/2 inch pieces) and add to the skillet. Cook for 5 minutes, until softened.
Season veggies with a sprinkle of salt and pepper.
Pour the egg mixture evenly over the vegetables.
Transfer skillet to the oven and bake for 25 minutes until puffed, golden, and cooked through.
Let rest for 10 minutes and garnish with parsley and shaved Paremesan.