In a large saucepan or Dutch oven over medium heat, cook the onions in the butter until soft, about 5 minutes. Add the fennel seed, red pepper flakes, garlic and orange zest and cook until fragrant, stirring, another 1 to 2 minutes. Turn the heat up to high and deglaze the pan with the wine, allowing it to bubble for a minute. Lower the heat to medium, add the stock and bring to a simmer. Add the prepped mussels and cover to steam until the mussels open, 3 to 4 minutes. Add the cream, stir until incorporated and remove from the heat. Serve immediately with sliced French bread for dipping.