In a large skillet, cook the bacon over medium-high heat until crisp, about 8 minutes. Using a slotted spoon, transfer to a plate. Discard all but 2 tablespoons of bacon fat from the pan. Add the beets and cook over high heat, stirring occasionally, until soft, about 5 minutes; season with salt and pepper.
Meanwhile, in a medium bowl, combine the mustard-mayonnaise and vinegar. Add the carrots and toss to coat; season with salt and pepper.
Spread 1 side of each bread slice with the beets, reserving 1/4 cup. Top 4 of the slices with the carrot mixture, watercress and another bread slice, beet side down. Spread the reserved beets on top. Divide the turkey and bacon among the 4 sandwiches, then top with the remaining bread.