Coat a 25 cm non-stick frying pan with non-stick cooking spray. Heat over medium heat. Beat eggs, egg white, milk and salt in a medium bowl until combined. Add broccoli. Pour into pan, spreading evenly. Cook, without stirring, until the mixture starts to thicken slightly around the edge, about 1 minute. Run a thin spatula around the edge of the pan, lifting mixture so the uncooked portion flows underneath the cooked one. Cook until centre is still moist but not runny, about 3 minutes.
Arrange tomato slices over one half of omelette. Top with cheese. Fold omelette over to cover filling. Reduce heat to low. Cover; cook until the egg component is set but still soft, 3 to 4 minutes. Slide omelette onto serving plate.