Cook the bacon in a medium nonstick skillet over medium-high heat, stirring occasionally, until crisp, 6 to 8 minutes. Set aside 1 1/2 teaspoons of the drippings. Add the leek to the skillet with the bacon and cook until softened, 3 minutes. Season with salt and pepper and set aside.
Combine the milk, 1 tablespoon butter and the mustard powder in a medium saucepan over medium heat and stir until the butter melts. Gradually add the cheese; whisk until melted and smooth but do not boil. (The sauce may look separated.) Add the egg yolks and cook, whisking, until slightly thickened, 3 to 5 minutes. Remove from the heat and stir in the Worcestershire sauce; season with salt and pepper.
Toast the bread and spread with the remaining 1 tablespoon butter; cut in half. Top with the bacon-leek mixture, then cover with the cheese sauce. Toss the watercress with the vinegar and the reserved drippings (rewarm if needed); season with salt and pepper. Serve with the toast.
Photograph by Antonis Achilleos
Courtesy of Food Network Magazine