Build individual open-faced sandwiches on a sheet pan like this:Spread each piece of party bread with a very thin layer of dijon mustard. Divide the turkey among the pieces of bread, cutting pieces to fit as needed. Season the turkey with salt and pepper. Put a slice of tomato on each sandwich. Cover the tomato with a thin slice of Velveeta.
Broil the Hot Browns until the cheese is melty and gooey. Remove from the oven. Garnish with crumbled bacon, chopped parsley and parmesan if using.