Clean and slice the mushrooms. Dice the onion and tomato. Finely chop the garlic.
Put a big skillet on medium heat and add olive oil. Once the oil is hot, add the onion and garlic. Sauté for 2 minutes (until onion becomes translucent).
Add the mushrooms and tomatoes, along with salt and pepper. Increase the heat to medium high (so the mushrooms do not get soggy). Cook until all the liquid has gone (about 5 minutes). If the mushrooms start sticking to the bottom, decrease the heat to medium low.
Transfer to a serving bowl and garnish with freshly chopped coriander. You can add this mixture to beaten eggs for making mushroom omelette.