Heat a cast iron skillet and add 2 Tbs olive oil. Saute onion until transluscent and add potatoes. Season with salt and
pepper and stir to coat with oil. Cook over medium heat for 5-8 minutes, without stirring. Stir and continue to cook until
potatoes soften and start t o brown. Remove potatoes and onions from pan and set aside.
Whisk 5 eggs together in a large bowl, add milk and cut up sausage. Add potato and onion mixture to eggs, stirring to incorporate. Wipe pan out and add 2 Tbs olive oil and 1 Tbs butter. Heat oil and butter until bubbly, swirling to coat up sides of pan.
Pour egg mixture into pan, and level out top with spatula. Cook over medium flame for 5-8 minutes, until eggs start to pull away
from sides of pan. Invert pan onto a large plate and slide frittata back into pan, cooked side up. Sprinkle with cheese and cook for 5 more minutes (can cover pan for last 2 minutes to melt cheese). If you are not comfortable flipping the frittata you can put it in a 350 oven for 5 mins, then sprinkle with cheese and return to oven for about 2 more mins