Wash off and debeard 2 pounds of mussels, discarding any dead ones. While wet, put them in a covered pot and cook on medium-high heat until they have opened; discard any that remain closed. Remove meat from all but 12 of the mussels, reserving the meat and the 12 fully-intact mussels. Strain liquid through a coffee filter to remove any sand and reserve that, too. Do this in advance of making the soup, for up to 2 days.
In a soup pot, saute a chopped onion in a little olive oil. Season with 1/2 teaspoon salt, a few grinds of black pepper and some whole fennel seeds. While that’s cooking, trim off the stalks from 2 fennel bulbs, reserving a few of the wispy fronds for a garnish. Dice the fennel bulbs and mince a clove of garlic. Open a 28-ounce can of plum tomatoes; remove seeds, chop tomatoes and reserve liquid.
When the onions are soft (maybe 5-10 minutes), stir in the fennel bulbs, garlic, 2 bay leaves, 1 quart of chicken stock, the tomatoes and tomato juice, the mussels (both the one in and out of their shells) and the mussel liquid, 1/2 cup of clam juice and 3 tablespoons of Pernod. Simmer for 10 minutes and check seasoning.
Serve in bowls, placing 2 or 3 of the mussels (in their shells) on the top. Garnish with fennel fronds. This makes 4 to 6 servings.