Heat the olive oil in a pan. Add the onion and cook, stirring, for about 30 seconds.
Add the chopped tomato, garlic and mushrooms. You may add chopped parsley if you like. Cook, stirring, for about a minute.
Pour in the egg-cream mixture. Season with salt and pepper. Keep stirring the mixture in the pan so that the eggs fluff up and no crust forms on the bottom. As soon as the eggs are partially set, turn off the heat. Scrambled eggs are best when the mixture is just a bit still runny.
Fill the bottom halves of the toasted English muffins with the omelet. Top with the upper halves. Serve with sliced onions and tomatoes on the side.