Melt butter in a nonstick skillet set over medium–high heat; cook mushrooms, green pepper, onion and ham, stirring often, for 3 to 5 minutes or until browned. Reduce heat to medium.
Whisk pepper into eggs. Spread mushroom mixture evenly in the skillet. Pour egg mixture evenly over mushroom mixture. Cook for 5 to 6 minutes or until set and golden on the bottom. Run a spatula around the sides.
Slide omelette onto a plate; flip and return to the skillet for 30 seconds to brown the other side. Cut into 6 wedges.
Stack 3 wedges, alternating with 3 tomato slices, onto each serving plate