Preheat oven to 425. Line a 7x11 or 9x13 dish with 2 layers of foil.
In a small bowl, whisk together Dijon mustard, syrup, rice wine vinegar, salt and pepper
Place chicken breasts in dish and pour the mixture on top. Turn the chicken around and over to make sure it all gets coated.
Bake, uncovered 40 minutes
Remove from oven and let chicken sit for 5 minutes. Transfer to a plate. Do not discard liquid
Immediately after plating chicken, whisk in 1 Tbsp cornstarch into the liquid in the plan. Serve sauce with chicken, garnish with rosemary.