Preheat broiler.
In 7-inch cast iron pan, heat oil and sauté onions and peppers over medium heat for 10 minutes or until veggies are caramelized. Make sure to scrap the bottom of pan with wooden spoon to get all the tasty brown bits!
While veggies are cooking, whisk eggs and salt in medium mixing bowl.
Add sausage to veggies and mix in pan.
Reduce heat and pour eggs over sausages and veggies in pan. Cook 5 minutes or until a shallow layer of uncooked eggs remain on top.
Sprinkle with cheese (if using) and oregano. Transfer pan to broiler and cook until top is cooked and beginning to brown, about 2 minutes.
Allow frittata to cool in pan a few minutes until slicing into wedges to serve.
