Heat oil in a large non-stick skillet on medium. Add onion, season with ¼ teaspoon each salt and pepper and cook, stirring occasionally for 4 minutes. Add mushrooms and cook, stirring occasionally until just tender, 4 minutes. Stir in spinach and cook until beginning to wilt.
Add eggs and cook stirring for 1 minute, then cook without stirring until edges are brown, 2 to 3 minutes. Sprinkle with cheese and fold half over the other to create a semi circle.
Toss tomatoes with parsley and avocado and serve spooned over omelette.
