This is a richer tasting fish stock.
Heat in a stockpot over medium heat:
      2 tablespoons butter
Add and cook, stirring occasionally, until they begin to soften, about 5 minutes:
      1 small onion, sliced
      1 large leek, white and tender green parts, cleaned thoroughly and sliced
      1/2 bulb fennel, sliced (optional)
      1 to 2 cloves garlic (optional)
Add and continue to cook:
      2 pounds fish heads and fish bones, or whole fish, scaled, gutted, gills, and viscera removed, rinsed well and drained
Cook, stirring once or twice, until they begin to turn opaque, 5 minutes more. Be sure not to let the vegetables or fish brown. Add:
      1 cup dry white wine
      6 cups cold water (or just enough to cover)
      1 bouquet garni
Bring to a boil, reduce the heat, and simmer gently. Cook, uncovered, skimming often, for 30 to 40 minutes. Strain into a clean pot or heatproof plastic container. Let cool, uncovered, then refrigerate until ready to use.