Soak the fish bones in a little cold water for 15 to 20 minutes. Strain and chop the bones into small pieces.
Peel and slice the onion.
Heat a large saucepan or cooking pot and add a little olive oil. Add the chopped onion and the fish bones to the pan. Cook without allowing the onion to colour. Add the white wine and then cover with water. Simmer for about 20 minutes.
Strain the liquid and then return 200ml of broth to the pan and reduce by half. Set aside the rest of the broth - this will be used to cook the ravioli later.
Wash and pick the spinach then chop finely.
Shuck the oysters and tip out the liquid into a bowl. Set aside.
Brush the ravioli sheets with a little beaten egg yolk. Place an oyster and a tarragon leaf in the centre of each sheet of ravioli. Sesaon with salt and pepper then place a second sheet of ravioli over the top. Press the pasta edges together, pushing out any excess air with the tips of your fingers. Use a pastry cutter to trim the edges of each ravioli.
Bring the unreduced broth to a vigorous simmer and then add the raviolis. Simmer for 1minute 30 seconds maximum.
Over a low heat, mix the liquid from the oysters with the cream and the reduced broth. Use a blender to whisk the sauce to a foam.
Serve the raviolis on a bed of spinach. Finish with a little oyster foam, some parmesan shavings and a pinch of piment d'Espelette.
«You can use any kind/size of oyster, choose your favourite and just make sure they are fresh!»