Preheat oven to 400 degrees (F). Spray a 12-inch non-stick oven-safe skillet pan with non-stick cooking spray. Heat pan over medium heat. Cut the Portuguese sausage into small pieces. Add the sausage to the pan and cook for 5 minutes. While cooking, separate the egg whites from the yolks and discard the yolks. Place the egg whites in a bowl, add a pinch of salt and black pepper, and whisk the egg whites together for 30 seconds or until foamy. Slice up the onions and mushrooms. After the Portuguese sausage is cooked, blot away the excess oil on the pan with a paper towel. Add the onions and mushrooms and cook for 1-2 minutes. Now add the egg whites to the skillet and place the cheese on top (do not stir the eggs). Place the skillet into the oven and bank for 15-20 minutes. Complete the dish by dividing the frittata among plates evenly. Serve with toasted English muffins or whole-wheat toast.