Mushroom, pepper and cream sauce

This goes with steak or magret de canard - it would probably also go with salmon, or anything you fancy, really....

1 small or 1/2 large onion
1 clove garlic
1/2 punnet (c 125 g) button mushrooms, or 2 large field mushrooms
1 large tablespoonful crème fraiche
1 tsp each black and Szechuan peppercorns
2 tsp cognac, armagnac or Calvados (optional, but does add to it!)
A little butter for frying

Chop the onion, crush the garlic, and halve the mushrooms if they are small; larger ones can be cut into relevant-sized pieces. Fry the onions and garlic gently in the butter until transparent, then add the mushrooms and continue to cook until they release their juices. Meanwhile, crush the peppercorns coarsely in a pestle and mortar. Now add them, and the remaining ingredients, to the sauce, bring to the boil, and serve at once.