4 slices bacon, chopped
1 leek (white and light green parts only), sliced and well rinsed
Kosher salt and freshly ground pepper
1/2 cup milk
2 tablespoons unsalted butter, at room temperature
1 teaspoon mustard powder
2 cups grated extra-sharp cheddar cheese (about 8 ounces)
2 large egg yolks
1 teaspoon Worcestershire sauce
6 thick slices sourdough or other crusty bread
1 bunch watercress, trimmed
2 teaspoons red wine vinegar