Whisk together Dijon mustard, honey, olive oil, vinegar, garlic, paprika, salt, and pepper in a small bowl.
Add chicken to a zip-top bag or shallow dish and pour in 2/3 of the marinade. Save the rest in the fridge.
Seal and refrigerate the chicken for at least 30 minutes, or overnight if you've got time.
Preheat grill to medium-high heat and lightly oil the grates.
Grill chicken for 6–8 minutes per side, brushing with reserved marinade during the last few minutes.
Cook until chicken reaches 165°F internally and has those perfect grill marks.
Let it rest for a few minutes before slicing. Garnish with fresh parsley if you like.