Saute the shallots and garlic in a small nonreactive saucepan, for 1 minute. Season with salt and pepper. Add the wine and bring to a boil. Add the crawfish and reseason. Bring back to a boil. Stir in the reduction and bring back to a boil. Reduce the heat and simmer, skimming off the fat and impurities several times for about 10 minutes.. Turn up the heat to high, skim the remaining impurities from the top of the sauce, cook for 1 to 2 minutes. Mount in the butter until all is incorporated. Add the green onions and remove from heat. Place a small amount of the mashed potatoes in the center of the plate, set the filet right on top and spoon the sauce over the filet. Garnish with shaved green onions and fresh black pepper.