Make this quickly in the same skillet you have cooked fish in and serve it over the fish—or make it separately to use as a sauce for green vegetables.
Melt in a small skillet over medium-low heat:
      4 to 5 tablespoons butter, preferably unsalted
Cook slowly, shaking or stirring so that it cooks evenly. Watch as the butter begins to foam; it can burn easily. Remove the butter from the heat when it becomes light brown and smells nutty. Stir in:
      1 teaspoon white wine vinegar or fresh lemon juice
      Salt and ground black pepper to taste
Serve immediately.