Heat the olive oil in a large skillet over medium heat. Sauté the green onions and shallots until they're limp.
Add the wine and bring it to a boil. Reduce until most of the liquid is gone. Add the cream, saffron, salt, pepper, cayenne and ginger, and bring to a light boil. (Also add tarragon at this point if using dried.)
Add crawfish tail meat. Cook for two more minutes, agitating the pan, until the heat is distributed equally throughout the now-steaming crawfish.
Put the vol-au-vents into the preheated oven for about two minutes. Remove from the oven, and spoon the crawfish into a puff pastry shell, until crawfish and sauce run over the top and onto the plate.
