Heat 1/2 teaspoon oil in a large nonstick skillet coated with cooking spray over medium-high heat. Add green onions; sauté 1 minute. Add bell pepper; sauté 1 minute. Add mushrooms; cook 3 minutes, stirring frequently. Stir in tomato, 1/4 teaspoon salt, and black pepper; cook 30 seconds. Remove vegetable mixture from pan; cover and keep warm.
Place 1/4 teaspoon salt, parsley, eggs, and egg whites in a bowl; stir well with a whisk to combine.
Place 1/2 teaspoon oil and butter in skillet over medium-high heat until butter melts. Add egg mixture to pan; cook until edges begin to set (about 2 minutes). Slide front edge of a spatula between edge of omelet and pan. Gently lift edge of omelet, tilting pan to allow some uncooked egg mixture to come in contact with pan. Repeat procedure on opposite edge of omelet. Continue cooking until the center is just set (about 7 minutes).
Spoon vegetable mixture evenly over 1/2 of omelet; top vegetable mixture with cheese. Loosen omelet with a spatula; fold in half. Carefully slide omelet onto a serving platter. Cut omelet into 4 wedges; top with sour cream. Serve immediately.