In a medium-sized bowl, whisk together vinegar, ⅓ cup olive oil, salt, garlic, lemon juice, and rosemary. Add the chicken to the marinade, and turn to coat. Marinate at room temperature for 20 minutes.
Preheat oven to 425 degrees F. In an iron skillet or other oven-safe skillet over medium high heat, place 2 tablespoons olive oil. When oil is hot, remove only the chicken thighs to the skillet (reserving marinade), and cook them for 2 minutes on each side.
Toss grapes and onions in the reserved marinade, then empty the whole thing into the skillet with the chicken. Carefully place the uncovered skillet in the oven, and bake for 20 minutes.
Recipe by Autoimmune Wellness at https://autoimmunewellness.com/roasted-grape-chicken-thighs/