Poach the salmon by putting it in a saucepan of cold salted water and bringing to the boil. Once the water has boiled remove the salmon, cool, and flake with a fork to use later.
Heat the olive oil in a hot pan and sweat the onion, cumin and thyme until soft. Turn up the heat, add the mushroom and garlic and pour in a splash of water. Put a lid on the pan and cook for a minute. Add the basil and tarragon and cook for a further minutes.
Tip the contents of the pan into a colander and reserve the cooking liquid.
For the sauce melt the butter in a heavy saucepan over a medium heat. Remove from the heat and stir in the flour to make a smooth paste. Cook the paste over a low heat, stirring, until the flour turns straw-coloured. Remove from the heat. Add the cream, 20g of the cheese, the cooking liquid from the mushrooms and simmer for five minutes while stirring.
Fold the flaked salmon into the mushroom and onion mix and add a 1/3 of the sauce to keep the mixture moist. Fill each pancake with the filling mix and roll to form a wrap.
Line the pancake rolls up side by side in a lightly buttered gratin dish and pour over the remaining sauce. Sprinkle the remaining cheese over the top.
Bake in an oven at 200°C for around 5 minutes. Garnish with chervil.