Heat a large cast iron skillet over high heat for 5 to 8 minutes. Pat the steaks dry with paper towels.
Combine the fleur de sel and the cracked pepper on a small plate and roll the NY steaks top, sides and bottom in the seasoning. Press lightly to coat well. When the skillet is very hot, add the steaks and sear on all sides.
Transfer the steaks from the skillet to a cookie sheet pan and place in the hot oven for 5 to 10 minutes. Remove from the oven, cover the steak pan with aluminum foil and let rest for 10 minutes.
Meanwhile in the same skillet that you just cooked your steaks, without scraping, add the butter and melt over medium high heat. Add the mushrooms and sauté until all the moisture is released. Stir in the sherry and cook until the mushrooms are cooked thru and are golden brown but not chewy. Taste and season with salt and pepper if needed. Remove the mushrooms from the skillet and set aside covered to keep warm.
Do not wipe the skillet, add the olive oil and shallots. Cook over medium heat for 2 minutes. Add whiskey to deglaze the pan and cook until the whiskey evaporates and the shallots are tender. Stir in cream and simmer for 4 to 5 minutes, stirring occasionally until thickened. Add the mustard to the cream, blending well. Taste for seasoning and add more cream if you wish for a milder sauce.
Place the steaks in the middle of the plate and spoon mustard sauce around the steaks and top with the mushrooms. Sprinkle with parsley.