Preheat the grill for 10 minutes with lid closed to reach 350 degrees, or if using an indoor grill heat for 3-4 minutes on the chicken setting or to 350 F.
Mix soy sauce, olive oil, mustard, salt, spices and herbs in a bowl. Place the chicken thighs in the marinade and toss to coat each one. If desired, you can cover the bowl with cling film and leave for half an hour in the cold.
When the grill is hot, lay the chicken thighs next to each other on the grate. Close the lid and cook until the beep indicates Well-done on indoor grill. Or about 8-12 minutes (4-5 min. each side), flipped halfway thru on outdoor grill until the middle reaches 165 degrees F.
Remove, cover on a plate and allow to rest for 10 minutes before slicing.