The Melt the butter and add it to the onions, sliced ​​thinly and head fennel, cut into thin slices lengthwise. Boil until tender and add salt and pepper. Add beer, waiting for the sauce to a boil and add the cleaned mussels with a brush. Cover the pan is with a lid and boil slightly, until mussels start to open. When most are open, remove them from the sauce and pour it into a large serving plate. Mussels that have not opened discarded. Add to sauce grated lemon peel and cream, waiting to boil and leave about 5 minutes to boil. Pour sauce over the mussels. Serve with toasted bread.