Spray a medium nonstick skillet with cooking spray and heat over medium fire.
Add mushrooms and bell pepper and cook, stirring occasionally, until softened, about 5 minutes. Season with salt and pepper, transfer to a bowl and wipe out the skillet with a damp paper towel.
Spray skillet again with coking spray and heat over medium heat and add th egg and cook until the center starts to set, about 2 minutes.
Spoon the vegetables over omelet.
Lift with a spatula so the uncooked portion of egg can flow, transfer to two plates, and top each serving with diced tomato.