In a small bowl, mix the olive oil, Dijon mustard, lemon juice, Worcestershire sauce, garlic powder, onion powder, dried thyme and kosher salt.
Pat the chicken thighs dry with a paper towel. Add the chicken to a container and pour on the marinade. Use your hands to evenly cover the chicken with the marinade. If using skin-on chicken thighs, make sure to use your fingers to get the marinade under the skin. Cover and marinate in the refrigerator for at least 30 minutes or up to 8 hours.
Preheat a grill to medium-high heat (375°F to 450°F).
Grill the chicken thighs smooth side down until grill marks appear and it releases from the grates, 5 to 6 minutes for small thighs and 6 to 8 for large thighs. Flip and cook until the internal temperature is 165°F, 4 to 5 minutes.
Remove to a platter and cover with foil. Rest for 5 minutes for maximum juiciness. Taste and if desired, add a few more pinches of salt to taste.