Clean the mushrooms, finely dice the garlic or cut into very thin slices. Heat the olive oil, fry the garlic and rosemary briefly, then add the mushrooms and fry with them. Season to taste with balsamic vinegar and salt and allow to cool. Let it go for at least half a day, or even better a whole day. Garnish with chopped flat-leaf parsley. Goes well with every buffet as a starter. Also very tasty with a baguette as a small dish.