To prepare the mussels, scrub them well with a stiff bristle brush under cold running water. Remove beards if there are any. Soak for 10 minutes in lightly salted water then rinse. Discard any with broken shells, shells that are open and don't close if you lightly tap them, or otherwise damaged mussels.
Make the breadcrumbs - heat the 2 tbsp oil in a skillet until hot and add the breadcrumbs, ground fennel powder and a pinch of salt. Cook, stirring, until the breadcrumbs brown up. Remove from the heat quickly so they don't burn and set aside.
In a large stock pot with a lid, heat the butter over medium heat. Add the garlic, shallot, leeks and fennel and stir. Cook until translucent, stirring occasionally. Add the wine and bring to a boil. Boil until the wine is reduced by half, then add the chicken stock, thyme, cream, potato, and saffron and reduce to a simmer. Cook until the potato is almost tender, about 10 minutes depending on the size of your pieces.
Bring to a boil again and add the mussels. Reduce the heat to medium, cover, and cook for about 5 minutes, tossing the mussels occasionally. Check at 5 minutes, and if they are all opened, proceed. If a bunch are still closed, continue cooking a few more minutes. Add in the lemon and zest and taste for salt and add a bit more if needed.
Dish up some mussels and broth into a bowl and top with chopped parsley, fennel frons, and some of the breadcrumbs. Reserve some bread crumbs for adding while eating.