Spray a small nonstick skillet with cooking spray. Add oil and heat over medium-high heat. Add tomatoes and scallion and cook, stirring once or twice, until softened, 1 to 2 minutes. Place spinach on top, cover and let wilt, about 30 seconds. Stir to combine.
Pour in egg substitute, reduce heat to medium-low and continue cooking, stirring constantly with a heatproof rubber spatula, until the egg is starting to set, about 20 seconds. Continue cooking, lifting the edges so the uncooked egg will flow underneath, until mostly set, about 30 seconds more.
Sprinkle cheese, salt and pepper over the omelet. Lift up an edge of the omelet and drizzle the remaining 1 tablespoon water under it. Cover, reduce heat to low and cook until the egg is completely set and the cheese is melted, about 2 minutes. Fold over using the spatula and serve.