Season the chicken with salt and pepper. Place it into a baking dish.
Combine the mustard, maple syrup and rice wine vinegar in a bowl. Season with salt and pepper. Pour the mixture over the chicken thighs.
Bake in a preheated 400-degree F oven for about 45 minutes or until chicken is cooked through.
Remove the chicken to a serving plate. Pour all the liquid into a saucepot. Bring to a simmer and whisk in cornstarch. Cook until it thickens, about 1 minute. Pour the sauce over the chicken and garnish with rosemary. Serve.
