In a large heavy pot, crisp the bacon over medium-high heat for about 8 minutes. Set bacon aside, leaving 1 tablespoon of drippings in the pot. Sauté celeriac, fennel, garlic, thyme, and cayenne pepper for 4 minutes until softened. Deglaze with wine and cook for 2 minutes until slightly reduced. Stir in stock and cream. Simmer partially covered for 10 minutes until flavors meld and veggies are tender.
Simmer the ling and prawns in the soup, uncovered, for around 5 minutes, or until the fish and prawns are cooked through and opaque.
Finely chop the bacon you set aside. Ladle the soup into bowls and garnish with the bacon and fennel fronds before serving.