Over medium high heat, saute the onion, potatoes, and sausage in a large cast iron skillet coated with the olive oil. Stir occasionally. If necessary, you may drizzle a little extra oil if the potatoes begin to stick as they cook. If using mushrooms, I add them in the beginning. I like the mushrooms a little more well done and the other veggies more on the raw-but-warm side.
Meanwhile, in a separate bowl, beat the eggs with the milk, oregano, and salt and pepper to taste.
Once the potatoes/sausage mixture begins to brown, add the remaining vegetables and black olives, stirring to combine. Reduce the heat to medium and pour the egg mixture over top of the sausage/veggie mixture. Cook, inverting to incorporate and cook throughout, until eggs are done to your liking. I like the eggs to retain just a bit of moistness.
Remove from heat and sprinkle with feta. Serve over a toasted corn tortilla, if desired.
