Whisk together 1½ tsp each grainy mustard, Dijon mustard and lemon juice; 1 tsp liquid honey; and ¾ tsp chopped fresh rosemary. Set aside.
Sprinkle 1 lb (450 g) boneless skinless chicken thighs with pinch each salt and pepper.
Place chicken on greased grill over medium-high heat; close lid and grill, turning once, for 8 minutes.
Brush with mustard mixture; cook, turning once, until juices run clear when chicken is pierced, about 2 minutes.
Let stand for 5 minutes before serving.