Four people:
• 0.5 liters chicken stock (or equivalent high-quality stock)
• 1 chopped banana onion
• 1/2 cup chopped mushrooms
• about 25 g of butter
• 2-3 tablespoons apple cider vinegar
• truffle or truffle oil
• salt
• pepper
1. Reduce the fond to half by letting it simmer slowly.
2. Add the onion and the mushrooms to the stock, let simmer gently.
3. Melt the butter in a frying pan until it becomes golden brown. Separate the clear butter from the bottom layer, which should not be used.
4. Season the sauce with apple cider vinegar, the browned butter, truffle (or truffle oil), salt and pepper to taste.
5. If you want a clear sauce, strain out the onion and the mushrooms. Otherwise, leave them in.