Preheat oven to 450°F and place oven rack in the middle.

Combine 2 tablespoons olive oil, garlic, lemon juice, oregano, and 2/3 of the rosemary in a small bowl. Stir and set aside.

Combine butternut squash, 1 tablespoon olive oil, and parmesan cheese in a bowl, season with salt and pepper, and toss until evenly coated. Set aside.

Place split breasts in a cast iron skillet, arranging as best you can not to overlap or overcrowd. Season liberally with salt and pepper.

Pour/brush the olive oil/seasoning mix over the chicken breasts until evenly coated.

Place skillet in the oven on the middle rack and roast for 25 minutes. Remove skillet from the oven and add butternut squash to the skillet, arranging as evenly as possible around the chicken. It's OK if some are slightly under the chicken. Bake for an additional 25-30 minutes, or until chicken reaches an internal temp of 160-165°F. I've found giving the butternut squash a quick toss after it's been in for 15 minutes helps it cook more evenly.

Remove skillet from the oven and let chicken rest in the skillet for 5 minutes before serving. It will continue to cook during this time. The final serving temp should be 165 °F. A digital thermometer really helps! Serve sprinkled with the rest of the fresh rosemary all over.