In old-fashioned French slang, un oeil au beurre noir—“an eye with black butter”—is a black eye. Delicious against the light flavor of sautéed fish such as sole, cod, or skate.
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 dark brown and smells slightly burnt. Stir in:
      1 teaspoon white wine vinegar or fresh lemon juice
      Salt and ground black pepper to taste
Serve immediately.