1. Cut the fish into small cubes (or have it done by the fishmonger).
2. Boil 1 liter of water with the stock cube.
3. When the water is boiling, cook the fish cubes in it for 5 minutes.
4. Remove the fish cubes with a slotted spoon and set them aside for later.
5. In a saucepan, bring 50 cl of milk + 4 to 5 tablespoons of the fish cooking broth to a boil.
6. When the milk is boiling, remove from heat and pour in the béchamel mixture.
7. Meanwhile, drain and rinse the forest mushrooms.
8. Reheat the béchamel over very low heat. Add the mushrooms and fish cubes, let it warm up while stirring regularly for about 10 minutes, until the béchamel has thickened properly and has absorbed the flavor of the fish.
9. Season the mixture, and if it still seems too liquid, add a tablespoon of flour.
10. Preheat the oven to 180°C (350°F).
11. Arrange the vol-au-vent shells in a dish and fill them with the mixture. Keep the remaining mixture warm in the saucepan.
12. Bake for about 10-12 minutes, then serve the vol-au-vents by adding the remaining hot filling around them.