If the marrow bones are still bloody, place them in cold water for up to 3 hours to remove the blood.
Press the marrow out of the bones with your fingers and place in water with a dash of vinegar.
Cut the shallots and the garlic into fine slices with the red wine in a saucepan, season with salt and pepper and cook for 1.5 hours, then puree, season to taste and mix in the butter.
Preheat the oven to 240 ° C.
Place three marrow bones on a fire-proof plate, fill with the shallot puree, then place 4 snails on top and cover the whole thing with the marrow. Sprinkle with coarse sea salt and season with pepper from the mill. Gratinate in the oven for 10 minutes.