1. Trim tough outer leaves from fennel; mince feathery fronds to measure 2 tablespoons. Remove and discard stalks. Cut fennel bulb in half lengthwise, and discard core. Vertically slice bulb.
2. Cook bacon in a large Dutch oven over medium heat for 3 minutes or until crisp, stirring frequently. Add fennel bulb, potatoes, salt, and pepper. Cook 10 minutes or until fennel is lightly browned, stirring occasionally. Increase heat to high. Add beer, scraping pan to loosen browned bits; bring to a boil. Stir in mussels and green beans; cover and cook 4 minutes or until mussels open. Discard any unopened shells. Stir in juice. Divide mussel mixture evenly among 4 bowls, and spoon broth evenly over mussels. Sprinkle each serving with 1 1/2 teaspoons chopped fennel fronds.