In a medium bowl, mix together the meat, shallot, parsley and Worcestershire sauce; season with salt and pepper. Score the meat into 4 equal portions and then form into large patties. Heat a drizzle of EVOO in a large nonstick skillet over medium-high heat. When the pan is screaming hot, add the patties and cook for 4 minutes on each side for medium rare, 6 minutes on each side for medium well.
Meanwhile, place a medium skillet over medium heat. Melt 2 tablespoons butter in a drizzle of EVOO, add the mushrooms and cook until tender, about 6 minutes. Pour in the sherry and let it cook off, about 30 seconds. Season the mushrooms with salt and pepper and remove from the heat.
Stir together the mustard and remaining 1 tablespoon butter. Spread the Dijon butter liberally on the bun bottoms. Top with the watercress, a burger patty, a pile of mushrooms and a few chopped cornichons. Spread the mousse pate on the bun tops and set them into place.