For the fish stock, place all the fish stock ingredients into a saucepan and cover with cold water. Place on the heat and bring to the boil.
Reduce the heat and simmer for 25 minutes, skimming off any scum that sits on the surface. Strain into a container and use when needed (the stock can be frozen if necessary - make sure it has cooled completely before you place it in the freezer and that the container you use is suitable for freezing).
For the flambéed lobster, heat a sauté pan until hot, add the olive oil, shallots and garlic and cook for two minutes.
Add the tomatoes and sauté for one minute. Stir in the lobster and the brandy.
Tip the pan towards the heat so that the brandy is set alight, tossing the lobster to be covered by the flames. Once the flames have subsided, add the fish stock and simmer for two minutes. Stir in the butter and chervil, season with salt and freshly ground black pepper and serve at once.
