For the base:
Heat oil in a large stock pot over medium heat. Add the onion, leeks, carrots, celery, fennel and garlic and stir frequently until tender, about 8-10 minutes. Increase the heat slightly. Add the tomato paste and stir constantly for 5 minutes, being careful not to burn the tomato paste. Add the remaining ingredients and bring to a boil. Once stock comes to a boil, reduce to a simmer. Cover and continue simmering for 1 hour, stirring frequently. This stock may be made up to 3 days ahead of time. It also freezes well.
To complete the stew:
Heat the oil in a large sauce pot over medium-high heat. Season the fish with the House seasoning and add to the pot along with the mussels and garlic. Stir frequently for 6-8 minutes and deglaze with the wine. Continue stirring until half of the wine has evaporated, about 2 minutes. Add the Amalfi Fish Stew base and bring to a simmer. Serve hot.