Whisk together eggs, water and soy sauce. In an 8-inch nonstick skillet over medium-high heat, melt 1 teaspoon of the butter. Add mushrooms and sauté until soft and browned. Remove mushrooms and wipe skillet dry with a paper towel.
In the same skillet, melt remaining 2 teaspoons butter over medium-high heat. When butter starts to foam, pour eggs into center of pan and stir with a rubber spatula. When eggs begin to set, pull in from sides of the pan with the spatula, tilting the pan so that unset eggs flow underneath. When eggs are mostly set, shake pan to loosen omelet and spoon mushrooms on top. Fold omelet in half and slide or flip onto plate. Garnish with chopped parsley. Serve immediately.