Heat olive oil in an 8-inch or 10-inch non-stick skillet over medium heat. Add the red onions, red peppers and mushrooms and cook until crisp tender, about 3-5 minutes. Add the spinach and continue cooking until the spinach wilts, about 1 more minute. Transfer the vegetables to a small bowl and wipe down the pan.

Crack and beat eggs with water in a small bowl. Pour the egg mixture in the same pan. As eggs begin to set around the edge of the skillet, use a spatula to gently push cooked portions toward the center of the skillet. Tilt and rotate the skillet to allow uncooked eggs to flow into empty spaces.

When eggs are almost set, add the cooked vegetables to half of the omelet and add the cheddar cheese on top. Place a spatula under the un-filled half, and fold over.

Gently slide from the skillet onto a plate. Season with salt and pepper and serve immediately topped with fresh parsley.