Combine eggs, spinach, oregano and salt in a medium bowl. Beat until well blended.
Lightly coat a 9-inch nonstick skillet with cooking spray, heat over medium heat.
Pour egg mixture into skillet. Cook until bottom is lightly browned and firm, about 5-6 minutes.
With a spatula, flip the omelet to the other side and cook 3 minutes more. Transfer omelet to a platter. Sprinkle tomatoes and cheese on one half (and onions, if desired) and fold over other half to cover.
Let stand 1 minute to warm tomatoes and cheese. Cut in half.
Vegetarian, Ready in 15 minutes, recipe by Eggland's Best
Per Serving: Calories 176; Fat 10g (56.2% calories from fat); Protein 14g; Carbohydrate 4g; Dietary Fiber 1g; Cholesterol 358mg; Sodium 512mg.