Tarragon sauce with cider

Butter 200g | Onion 750g | Fresh tarragon 40g | Apple cider vinegar 240ml | Cider (or cava or white wine) 750g | Milk cream 1’750l | Fish stock 1’500l | Salt eq | Black pepper eq

Cut the onion into 0.5cm fine brunoise, poach the onion with the butter
Add the fresh tarragon leaves and fry for 10 more minutes
Add the apple cider vinegar, turn up the heat and let it reduce for 10 minutes
Pour in the cream and cook over a gentle boil
It is important to remove at this point to prevent it from seizing
In a separate saucepan, reduce the apple cider to a boil for 15 minutes and add to the previous mixture
Add the fish stock.
Cook over high heat, stirring constantly so that it does not stick and until you get a thick consistency.
Seasoning
Put it through Thermomix and then through a Chinese.