Rinse, pat dry and slice mushrooms (makes about 1 ¼ cups); set aside. In a medium skillet melt 1-T. butter. Add mushrooms and onion; sauté until golden, about 5 minutes; remove from skillet. In a mixing bowl combine eggs, salt, oregano and pepper.

In the skillet used to sauté mushrooms, melt remaining 2-T. butter. Pour in egg mixture. Cook over moderate heat letting uncooked egg run under the cooked portion. When almost set, top with mushroom mixture and spinach. Sprinkle with Parmesan cheese. Place skillet under a preheated moderately hot broiler until top of omelet is set. Turn out onto a plate and serve.