Preheat oven to 400℉. Lay puff pastry dough in a 8" cast iron skillet to cover the full pan, cutting the extra dough off the edges and adding it to empty places. Poke dough with fork and bake for 10-12 mins until golden brown. Use a spoon to flatten bottom and sides of crust after baking.
Prepare sausage in a medium skillet until thoroughly cooked through. Set aside to cool on a paper towel lined plate and set aside.
Prepare cheese sauce -- in a small sauce pan, melt butter until frothy and whisk in flour. Over medium heat, slowly whisk the whole milk in a little at a time so that it thickens. Once all added and sauce has thickened, gradually add shredded while continuously stirring, season with salt and pepper to taste and keep warm.
Cook your eggs -- whisk eggs and milk in a large bowl and cook over medium heat in a greased skillet. Once eggs are ¾ way cooked through, turn off heat and set lid over pan for eggs to finish cooking.
Assemble & bake-- pour cheese sauce ont ocrust, then eggs, remaining ½ cup pf cheese and finally top with the cooked sausage. Bake for 15 mins and let sit for at least 15 minutes before slicing and serving.