Break the eggs into a bowl, add water, season with salt and pepper and beat with a fork for 1 minute or until thoroughly combined.
Set a large non-stick pan over high heat, add the butter and tilt pan to coat the bottom and sides.
When the butter begins to foam and just begins to color, pour eggs into the middle of the pan. At the same time, shake and swirl the pan to distribute the eggs over the surface. Hold over the heat for a few seconds, or until a film of coagulated egg is formed on the bottom of the pan.
Remove from heat. Shape the omelette by folding the third of the circle nearest the omelette nearest the handle into the center. Tilt the pan away from you and slide omelette down.
Line the center with sauted mushrooms. Fold opposite edge into the center and turn out onto servingplate so that the center is thick and the ends are tapered.