Preheat your oven to 400°F (200°C).
Heat the extra-virgin olive oil in an oven-proof skillet over medium-high heat. Add the smashed garlic cloves and swirl them to season the oil.
Season the skin side of the chicken thighs generously with salt and pepper. Place them skin side down in the hot skillet. Season the other side of the chicken with salt and pepper as well. Remove the garlic cloves.
Sear the skin side of the chicken until it becomes golden and crispy, which should take about 6 - 7 minutes. Reduce the heat to medium if needed to avoid burning.
Flip the chicken thighs and place the sliced shallots between them. Drizzle with balsamic vinegar.
Top the chicken and shallots with the grape clusters, nestling them between and around the chicken pieces. Sprinkle with chopped rosemary.
Transfer the skillet to the preheated oven and roast for approximately 20 minutes or until the internal temperature of the chicken reaches 170°F (77°C).
Once the chicken seems done, return the skillet to the stove and bring it to a quick simmer to ensure the juices are cooked. Spoon some flavorful pan sauce over everything.