Peel and dice the onion. Scald the tomatoes with boiling water, peel them and also cut into small cubes.
Heat the butter in a pan and briefly sweat the onion cubes in it. Add mushroom slices and diced tomatoes. Salt and pepper. Let the vegetables simmer for about 5 minutes and possibly pour off some of the resulting liquid. Finally, cut the basil into small pieces and stir in. Season everything again to taste.
The vegetables should have a juicy consistency, i.e. the tomato cubes should disintegrate and result in a slightly liquid base.
Suitable as a side dish for grilling, raclette or cold on bread (but then a little less liquid!).