Heat the olive oil in a large casserole dish over a moderate heat. Sweat the onion, garlic for 10 minutes with a little salt, stirring occasionally until they are soft but not coloured. Stir in the tomato purée, fennel seeds, canned chopped tomatoes and bay leaf. Stir well and cover with the fish stock and water.
Bring to the boil then reduce to a simmer for 30 minutes. Strain into a large, clean saucepan, pressing the tomatoes and all other ingredients through the sieve to extract as much flavour as possible. Add the Pernod to the broth, then add the potato and sliced fennel and bring to a simmer.
Once it has reached a simmer, add the sardines. Cook gently for 10 minutes until the potato and fennel are tender. Adjust the seasoning as necessary, then remove from the heat and garnish with sprigs of thyme. Serve immediately.