Preheat a frying pan, pour in vegetable oil. Put the onion in hot oil. Fry over medium heat, stirring occasionally, until golden brown (3-4 minutes). Then add the mushrooms. Mix.
Fry everything together, stirring occasionally, for 3-4 minutes.
In the meantime, wash the tomatoes, cut them in half, cut out the stalk, and grate (if you rub the tomato from the side of the cut, the peel will remain on the grater).
Add grated tomatoes to the pan. Mix.
Simmer the mushroom ragout on the lowest heat for about 5 minutes.