Place the chicken thighs in a large bowl or ziplock bag and season with salt and pepper.
Whisk together the olive oil, Dijon mustard, soy sauce, minced thyme, garlic, parsley, lemon juice, and pepper flakes in a small mixing bowl.
Pour the marinade over the chicken and thoroughly mix until the marinade completely coats every inch of the chicken. Tightly cover your bowl with plastic wrap (if using Ziploc, seal the bag), and allow the chicken thighs to marinate for 30 minutes or up to 24 hours in the fridge.
Remove chicken from the marinade, shake off excess, and discard the remaining marinade. Preheat your grill to medium-high heat 375-400°F (190-204°C).
Place the chicken thighs on the preheated grill and cook for 5-7 minutes per side, or until the internal temperature of the chicken reaches 165°F/74℃. Use a thermometer for best results.
Remove the chicken thighs from the grill, cover lightly with foil, and let them rest for 5-10 minutes before serving. Enjoy!