Heat broiler, with oven rack set about 4 inches from heat. Line a baking sheet with aluminum foil.
Spread one side of each cutlet with mustard, dividing amongst all 8 cutlets. Top with spinach and brie cheese. Sprinkle with salt and pepper. Starting at the short end of the chicken, roll tightly and place, seam side down, on prepared baking sheet. Sprinkle with a little more salt and pepper.
Broil, without turning, until tops are lightly browned and chicken is cooked through, 8-10 minutes.