In a large saucepan, combine the stock, tomatoes, tomato paste, liqueur, garlic, and saffron. Bring to a boil, then lower the heat and simmer, uncovered, 30 minutes.
Strain the soup, pressing the solids against the strainer to extract all the liquid. Return the liquid to the saucepan and stir in the fennel. Simmer just until the fennel is crisp-tender, 2 to 3 m