To finish the bouillabaisse, heat the remaining 1 tbsp of olive oil in a wide Flameproof casserole over a medium heat. Cook the fennel and onion for about 5 minutes until the onion just softens, then add the stock. Bring to a boil, add the potatoes and cook for 5-7 minutes Reduce the heat to medium and add the fish, starting with the thickest pieces and adding the thinner ones after 2 or 3 minutes. Add the shrimp and continue simmering gently until all the fish and shellfish is cookes.