Heat 1 Tbsp olive oil in a skillet over MED-HIGH. Cook mushrooms and onion for 5 minutes, then add salt, pepper, and spinach until wilted. Transfer to plate.
Beat eggs with 1 Tbsp water, remaining salt, and pepper. Wipe skillet clean.
Add 1 Tbsp butter to skillet over MED to MED-LOW. Pour half of egg mixture, cook about 2 minutes until slightly shiny.
Add half of mushroom mixture and cheese to one side. Fold omelet once eggs are lightly golden.
Slide omelet onto plate. Repeat with remaining ingredients. Garnish with parsley and black pepper.
Notes
Use fresh ingredients for best flavor. Feel free to customize with additional herbs.