Heat a large non-stick saute pan. When the pan is hot, render the sausage for 2 minutes. Add the onions and peppers and continue sauteing for 2 to 3 minutes, or until the vegetables wilt. Add the garlic and saute for 1 minute. Season the eggs with salt and white pepper. Pour the eggs into the pan. Stir the eggs gently until they start the thicken. Using a rubber spatula, lift the edges of the omelet so the uncooked eggs run underneath. Continue cooking the omelet until it is almost firm on top, 30 to 45 seconds, and golden on the bottom. Flip the omelet and sprinkle the top with the grated cheese. Continue cooking for 1 minute or until the cheese starts to melt. Place the frittata on plate. Serve with the salsa. Garnish with the chopped green onions.