Preheat oven to 450 degrees. Line a 8x8" oven-proof pan with 2 layers of tin foil.
In small bowl, whisk together Dijon mustard, maple syrup, rice wine vinegar, salt and pepper.
Place the chicken thighs in the foil layered pan then pour the mixture on top of it. Turn the chicken around in the sauce to make sure it gets all coated.
Bake, uncovered, for 40 minutes.
Remove from oven and let chicken sit for 5 minutes then transfer to a plate. Do NOT discard the liquid.
Immediately after plating chicken, whisk in the 1 tbsp. of cornstarch into the liquid in the pan. You'll create a nice, thick sauce to drizzle over your chicken. You need to do cornstarch mixing immediately because the liquid needs to be hot in order for it to thicken properly. If it's still not thickening after 1 tbsp., you can add a little more.
Sprinkle rosemary on top before serving.
Serve with rice or potatoes or vegetables.