In a 3- or 4-qt. sauté pan, melt the butter over medium heat. Add the garlic, onion, fennel and fennel seeds and cook, stirring occasionally, until sliced fennel is softened, 5 - 6 minutes.
Stir in the wine and orange zest. Increase the heat to high and boil for 1 minute.
Add the stock and return the mixture to a boil.
Add the mussels and cover the pot. Cook over high heat for 4 - 5 minutes or until the mussels have opened.
Discard any mussels that do not open before serving. Season to taste with salt and pepper.
Divide the mussels into 4 bowls. Ladle some of the hot stock into each bowl and serve with crusty bread for dipping.