Slice the mushrooms and roughly cut the cherry tomatoes. To a non-stick pan over medium heat, add the mushrooms. Sauté for 4 minutes until the mushrooms release all their liquid and are slightly golden.
Then, add the spinach and allow it to wilt down. Ensure that all the liquid from the spinach has evaporated. Transfer the mushrooms and spinach to a plate and add the olive oil to the pan.
Beat the egg and egg white together with the salt and black pepper in a small bowl, then add to the pan with olive oil. Cook for about 1 minute until the top is set and barely runny. If you like your eggs dry, allow the top to be fully set.
Add the spinach and mushrooms on the top and fold the eggs from one side to slightly cover the mushrooms and spinach. Serve on a plate, and top with sliced tomatoes.