Melt one tablespoon of the butter in a medium skillet over medium heat. Add onion and cook until soft (2 minutes).
Add mushroom and cook another 2 minutes (or until mushroom is soft).
Add bell pepper and cook another 2 minutes (or until pepper is soft). Remove from heat and stir in parsley.
Remove vegetables from skillet and set aside.
Beat eggs with 1 tablespoon water and salt and pepper.
Melt remaining 1 tablespoon butter in an 8-inch non-stick skillet (or use the same skillet used to cook vegetables in). Pour in the eggs. Cook until edges begin to set- watch because it cooks fast (2-3 seconds).
With a silicone spatula, stir mixture gently in a circular motion until thickened (10 seconds). Push back egg (from the edges of the pan) that has set so uncooked egg can run underneath. After about 20-30 seconds the top will be still wet but not runny and the omlet will be done. Remove skillet from heat.
With a silicone spatula fold the lower third (closest to you) of the omlet to the center and then the far edge (1/3rd) towards the center.
Make a shallow slit the length of the omlet and spoon the vegetables onto it.