Preheat oven to 200C. Stud the fillet with the garlic halves.
Heat the butter in a flameproof casserole with walnut oil. Add the fillet and brown on all sides over high heat. Transfer to the oven and roast the beef for 20 minutes. Remove the beef from the oven, cover with foil and allow to rest in its own juices for 10 minutes.
Deglaze casserole with wine. Add the snails, shallot and parsley and combine. Allow the snail mixture to reduce for 10 minutes, then add the pastis and flambé the whole dish.
Heat the olive oil in a frying pan, add the sliced garlic and sauté until golden. Add the garlic mixture to the snails, and season with salt and pepper. Carve the fillet into thin slices and scatter over some of the snails and garlic mixture. Serve the rest on the side.
Note Canned snails are available from Simon Johnson and select