you ll have to leave them unfed for 40 days first so all the crap get out of their guts . then boil them gently till tender and put them back in the shells and cover with garlic butter . and in the oven .
Chris I know you're french but 40 days without food sounds excessive for a snail.
"Put them in a pen and feed them for at least 3 days to purge their system of any toxins and to give them a chance to die if they have consumed a lethal dose. If they are still healthy after 3 or 4 days, they should be O.K.. Withhold all food, except water, for the last 1 to 2 days."