Preheat oven to 450ºF. Mix 1/2 cup of dijon mustard, 1/4 cup of maple syrup, & 1 tablespoon of rice/rice wine vinegar. Put a 1.5 pound package of boneless, skinless chicken thighs into a foil-lined, oven-proof baking dish. Salt/pepper thighs. Fully coat w/maple mustard mixture. Bake thighs for 40min or until meat thermometer reads 165ºF. Baste tops of thighs w/more sauce 1/2way through. Let thighs rest for 5min. Plate the chicken, spoon extra sauce over the top. Sprinkle fresh rosemary.