Combine all ingredients aside from the chicken in a small bowl and mix to combine.
Add the chicken and toss to coat evenly.
Grill Chicken on a hot grill or broiler for about 6-8 minutes per side, until just starting to brown and cooked through. Be careful not to over-cook it, or the chicken will be dry.
Recipe by Overtime Cook at https://overtimecook.com/2014/05/15/grilled-chicken-mustard-sauce/