In a large bowl, whisk together the apple cider vinegar, Dijon mustard, finely chopped garlic, lemon juice, brown sugar, dried minced onion, dried parsley, kosher salt, freshly ground black pepper, and olive oil until the mixture is well combined and emulsified.

Place the chicken breasts or thighs into the marinade, turning each piece to ensure full coverage.

Cover the bowl and refrigerate for at least four hours, or preferably overnight, to let the flavors infuse.

Preheat your grill to 400°F (204°C).

Remove the marinated chicken from the refrigerator and let it sit at room temperature for about 10 minutes.

Place the chicken pieces on the grill, then reduce the heat to medium.

Grill for 8-10 minutes on each side, depending on the thickness of the chicken, until fully cooked and the internal temperature reaches 165°F (74°C).

Transfer the grilled chicken to a serving platter.

Top each piece with the softened butter, turning the chicken to coat and allow the butter to melt and enhance the flavor.

Garnish with a sprinkle of extra dried parsley if desired.

Cover the platter with foil and let the chicken rest for five minutes before serving.