Season the steaks liberally with salt and 5-peppercorn mix. On a hot grill, mark the steaks and cook each side about 4 to 5 minutes. I prefer medium rare, which takes about 8 minutes total. Make sure to let the steaks rest after grilling for about 3 minutes to let the meat relax.
For the mustard oil: In a bowl, whisk mustard, turmeric and salt with water to form a paste. Whisk in oil and let stand overnight. Mixture will separate, don't re-mix before using, you want to use the clear flavored oil off the top.
For the shallot sauce: In a 1 quart stainless steel saucepan, add 1 tablespoon of butter and caramelize the shallots. Season. Add the red wine and reduce until almost dry. Add the mustard and saute for 2 minutes then add the cream. Heat the cream until hot, but not boiling. Pour into a blender and blend at high speed. Add the remaining butter and check for seasoning. Keep warm in a water bath.
For the gratin: Pre-heat the oven to 325 degrees. In a large bowl, mix everything together. Taste the cream for proper seasoning. Dump contents in a baking dish with at least a 2-inch lip. Make sure the cream covers the mixture. Bake in oven for 30 minutes, then stir well. Bake another 45 minutes until cream is thickened and potatoes are soft.
Cut the gratin with a ring mold and place in the center of a plate. Spoon shallot-pommery sauce around the gratin. Slice the steak and encircle the gratin. Garnish with crispy onions and mustard oil.