How to Make It
In a medium bowl, blend the butter with half of the garlic, 1 tablespoon of the lemon juice, the honey, horseradish and chopped parsley. Season generously with salt and pepper.
In a small saucepan of salted boiling water, blanch the parsley leaves until bright green, about 1 minute. Drain and cool under running water. In a blender, puree the parsley with 2 tablespoons of water and the remaining garlic and 1 tablespoon of lemon juice until nearly smooth. With the machine on, gradually add the 1/3 cup of oil until incorporated. Season the parsley sauce with salt and pepper.
Light a grill. Brush the chicken thighs with oil and season with salt and pepper. Grill over moderate heat, turning occasionally, until charred in spots and just cooked through, 20 to 25 minutes. Brush the chicken all over with the honey butter and grill, turning and brushing, until glazed, about 2 minutes more. Transfer the chicken to a platter or plates and serve with the parsley sauce.