Preheat the oven to 375 degrees . Arrange the bacon on a broiler pan and bake until crisp, about 15 minutes. Let cool, then chop into 1-inch pieces. Wipe the pan.
Preheat the broiler.
In a large saucepan, melt the butter over medium heat. Sprinkle with the flour, then whisk 1 to 2 minutes, being careful not to brown the flour. Stir in the Worcestershire sauce, mustard and hot sauce. Add the beer; whisk until the mixture thickens, about 1 minute. Reduce the heat to low and add the cheese. Whisk until smooth.
Arrange the toast on the broiler pan; top with the watercress and tomato slices. Spoon some cheese sauce on top and broil until the sauce bubbles and browns. Sprinkle with parsley and top with the bacon.