In a medium-sized bowl whisk together honey and mustard. Pour 1/3 of the mixture into a small bowl or container and reserve to the side.
Add garlic to the mixing bowl, stir to combine.
Cut a few horizontal slits in each chicken breast being careful not to cut too deep. Add chicken to the bowl, turn a few times and rub marinade into the chicken slits. Let the chicken marinate for at least 20 minutes. Grill then brush with the reserved sauce.
Preheat the grill and lightly oil the grill grate. Cook the chicken on the preheated grill over medium-high heat for about 20 minutes, turn with tongs midway through cooking. Close the lid to the grill to help speed up the cooking time and create a golden crust on the chicken.
Pour or brush the reserved marinade over each piece of chicken about 10 minutes, before the chicken is cooked through to 165ºF internal temperature.