1. Scrub the mussels under cold water, drain them. Remove the core of the fennel and peel the outer layer to eliminate the strings. Cut it into quarters lengthwise, rinse the quarters and finely slice them. Peel and press the garlic.
2. Heat the oil in a large pot, add the fennel and cook for 10 minutes without browning. Add the garlic and mix. Add the mussels, cover and increase the heat. Stop cooking when all the mussels have opened, then drizzle with the juice of the squeezed lemon. Season with pepper, mix well and serve immediately.