Whisk the Dijon and minced garlic together until smooth. Slowly stream in the olive oil while whisking so the marinade emulsifies and clings.
Whisk in the balsamic, Worcestershire, honey, rosemary, thyme, salt, pepper, and red pepper flakes.
Add the chicken to a zip-top bag or shallow dish and pour the marinade over. Toss to coat. Refrigerate 4 to 8 hours for thighs, or 2 to 4 hours for breasts.
Heat one side of the grill to medium-high and leave the other side off. Clean the grates, then oil them just before cooking.
Lay the thighs smooth-side down over the hot zone. Do not move them for 4 to 5 minutes, until they release easily, then flip.
Cook the second side 4 to 5 minutes, until grill-marked and caramelized. Pull at 175°F, sliding them to the cool side if they darken too fast.
Rest 5 minutes before serving, then top with minced chives.
Video
Notes
Chicken breasts: pound to an even thickness, marinate 2 to 4 hours, and grill to 165°F.Reserve marinade: set some aside before it touches raw chicken to drizzle at the end.Nutrition: values are an automated estimate.