Marinate the fish steaks in olive oil and garlic for 1 hour at room temperature. Remove the fish from the marinade and pour the marinade into a large saucepan.
Heat the reserved marinade, add the onions and cook them until soft but not brown. Add the stock, potatoes, tomatoes, bouquet garni, orange zest, fennel, salt and pepper. Bring to a boil and simmer, skim