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.
Yield: 4 servings