Every one likes this sauce for either meat or fish. In a double saucepan melt a lump of butter, flavor it with salt, pepper, some minced parsley that you had first rubbed on a raw slice of onion, and some lemon-juice. Use vinegar instead of the lemon if you wish, but do not forget that it does not require so much vinegar. Mix it with a fork and serve it warm; do not let it bubble.