Heat broiler. If using a broiler-proof rimmed baking sheet, set rack 4 inches from heat; if using a broiler pan, set rack so that top of pan is 4 inches from heat. On sheet (or pan), arrange potatoes and mushrooms in a single layer. Sprinkle with bacon, sage, 1 teaspoon salt, and 1/4 teaspoon pepper.
Broil, tossing every 5 to 6 minutes, until potatoes are tender and browned, 20 to 25 minutes. Transfer to a serving bowl (reserve sheet).
Place turkey cutlets on sheet; season with salt and pepper, and sprinkle with Parmesan, dividing evenly. Broil until cheese is golden and turkey is opaque throughout, 6 to 8 minutes.
Meanwhile, toss potatoes and mushrooms with parsley; season with salt and pepper. Keep warm.
Serve turkey cutlets with potato-mushroom mixture, accompanied by grainy mustard.
Parmesan cheese helps brown the cutlets and keeps them moist during broiling.