Preheat oven to 425 degrees F.
Place the chicken, onion, garlic and grapes in cast iron skillet. Pour the olive oil over the ingredients and massage into chicken, grapes, onion and garlic. Arrange ingredients in pan so that chicken is on the bottom, onion and garlic are nestled amongst chicken thighs and grapes are dispersed over the top of chicken.
Pour in the wine, if desired. Spread the rosemary, salt and pepper over the entire dish.
Roast dish for about 25-30 minutes, or until temperature (measured in largest thigh) registers 165 degrees F. Move cast iron skillet to top of the oven and broil for 2-3 minutes, until grapes and chicken begin to brown.