Cooking the fish is done in "court bouillon" that you can make yourself or based on powdered/cube vegetable broth.
For seafood you could also use a frozen mix (not tried).
For the sauce, make a béchamel with the juice from the mussels and top up with the poaching broth for ~0.5 l.
You can also add a little saffron.
Color the sauce with a little tomato purée or sweet paprika or saffron. I used canned chanterelles because they were out of season. So I used some of the water from the chanterelles for the béchamel sauce.
For 4 people as an aperitif (first course) - double the recipe for the main course.
100 g mussels (probably in a jar) otherwise cooked "marinière"
60+ g small cooked prawns (or peel and cook)
6 scallops (~140 g)
100 g filet of cod
100 g mushrooms or chanterelles
1 shallot, finely chopped (only if fresh mushrooms)
1 dash of lemon (only if fresh mushrooms)
saffron or sweet paprika
1 dl white wine
4 vol-au-vent shells
Prepare 2 bowls, one with the seafood, the other for the bechamel liquid.
Put ~100 g of the cooked mussels in one bowl, and the mussel juice in the other.
Cut the mushrooms if there are any, sauté the shallot, then fry the mushrooms with a little lemon juice, salt, set aside with the seafood.
Pan-fry the scallops, salt, deglaze with the wine and cut the scallops into 4, set aside with the mushrooms. Reserve the reduced wine with the liquid.
The small prawns are probably already cooked if not quickly pan-fried after the scallops. Cut them up and add them to the mixture.
Poach the cod (not too much so that it doesn't crumble ~5 min). Cut it up and add it to the mixture.
Top up the liquid with enough poaching broth to obtain ~0.4 l of liquid.
Prepare the béchamel with 30 g of butter, 30 g of flour, and 4 dl of liquid.
Add 1 tsp of tomato purée, a few sprigs of saffron or the paprika, S + P, a little grated nutmeg and 2 pinches of sugar.
Mix with seafood, reheat carefully, no boil so as to not overcook the seafood, check seasoning, add chopped parsley.
Heat the shells and serve.