Preheat the grill or broiler.
Rub the steak on both sides with the oil and season with Essence, salt, and pepper.
Place on the grill (or on a broiler pan) and cook until medium-rare, about 8 minutes per side. Remove from the grill and let rest for 5 minutes. Slice thinly on the diagonal.
While the meat is cooking, in a saucepan, melt 1 tablespoon of the butter over medium heat. Add the shallots and cook, stirring, for 3 minutes. Add the wine, bring to a boil, and cook until reduced by 1/2. Add the thyme and beef stock and bring to a boil. Simmer until reduced by 1/2. Lower the heat and whisking, add the remaining butter 1 piece at a time. Remove the thyme and stir in the marrow. Remove from the heat and adjust the seasoning, to taste.
Arrange the meat on 4 plates and drizzle with the sauce. Garnish the plates with watercress and serve with Blue Cheese Mashed Potatoes.
Combine all ingredients thoroughly and store in an airtight jar or container.
Recipe from "New New Orleans Cooking", by Emeril Lagasse and Jessie Tirsch. Published by William and Morrow, 1993.
Peel and quarter the potatoes lengthwise, then cut into 1-inch wedges. Put in a heavy medium saucepan with 1 teaspoon of the salt and add enough cold water to cover by 1-inch. Bring to a boil over high heat. Reduce the heat to medium-low. Simmer until the potatoes are fork-tender, about 25 minutes.
Drain in a colander. Return the potatoes to the saucepan. Add the cream, butter, the remaining 3/4 teaspoon salt, and the pepper. With the heat on medium-low, mash and stir the mixture until well blended, 4 to 5 minutes. Mix in the blue cheese and parsley and mash to incorporate.