Brown sausage in a skillet over medium-high heat, breaking up with a spoon to crumble. Cook until no longer pink. Add in the green chiles and potatoes o'brien and cook until heated through. Remove the sausage mixture from heat.
Lightly grease a 9"x13" casserole dish with non-stick spray and set aside. Place about ⅔ cup of the sausage mixture on top of a flour tortilla and add 2 tbsp of shredded cheese. Roll up the tortilla, keeping the ends open, then place seem-side down in the casserole dish. Repeat with the remaining filling and tortillas. Sprinkle any remaining filling over the top of the rolled tortillas as well as the remainder of the shredded cheese.
Whisk together the eggs, enchilada sauce, milk, salt and pepper until it's a uniform color. Pour the egg mixture over the top of the rolled tortillas. Cover with plastic wrap and place in the fridge for up to 24 hours. (or you can bake right away if desired)
Preheat oven to 350 degrees F. Remove plastic wrap from casserole dish and cover tightly with foil. Bake for 35 minutes, then remove the foil and bake an additional 10 minutes, or until the eggs are set.