Whisk together the olive oil, lemon juice, Dijon mustard, garlic powder, salt, and pepper in a medium bowl.
Place chicken in a zip lock bag and pour the marinade on top. Make sure each piece is coated.
Marinate for at least 30 minutes or refrigerate for up to 2 hours.
Heat a propane grill to medium heat.
With a paper towel dipped in olive oil, oil the grates. Add the chicken.
Grill covered for 7 minutes.
Flip and cook for another 7 minutes, or until cooked through.
Allow to rest for 5 minutes.
Optional: Grill lemon slices to use as a garnish when serving, along with some fresh rosemary.