Preheat oven to 450 degrees Fahrenheit. Line a 8x8" oven-proof pan with 2 layers of tin foil.

In small bowl, whisk together Dijon mustard, maple syrup, vinegar, salt and pepper.

Place the chicken 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 cornstarch into the liquid in the pan to create a thick sauce. 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 more.

Sprinkle rosemary on top before serving.

Serve with rice or potatoes or vegetables.