Combine the mustard, avocado oil, honey, salt and water in a small bowl and stir well until smooth. Add the chicken to a medium glass container and pour marinade over it, making sure all sides are coated. Let marinate for at least 1 hour or up to overnight.
If only marinating an hour, keep at room temperature. If marinating overnight then cover and place in the fridge. Remove 1 hour before grilling.
Grill over medium/high heat for 8 minutes, flipping half way or until 165° internally. Discard any extra marinade.