In a sauce pan on medium low heat melt butter and add onions and mushrooms. Stir until onions are soft but not browned. Add spinach and stir until it is heated through. Immediately remove from heat and set a side.
In a mixing bowl add eggs, water and salt beat and stir well. Set aside.
Heat a 12" fry pan on medium low heat, add a little oil (1 tsp.) or spray with a non stick vegetable spray. A non stick pan works great. Place egg mixture in pan and sprinkle with tomato and 1/2 of the shredded cheese. Place a lid on until omelet starts to set. Immediately remove lid and fold omelet from the sides to the middle. If this is difficult fold in half. sprinkle with the rest of cheese.
Serve with a side order of holondaise sauce.Makes 1 Source: