Arrange racks in bottom and top thirds of oven, then heat oven to 450°F.
STEP 2
Place chicken, rosemary, and garlic on a rimmed baking sheet. Add lemons to the center.
STEP 3
Brush chicken with 1.5 tablespoons oil and sprinkle with 0.75 teaspoon each salt and pepper; roast in top third of oven until golden and instant-read thermometer registers 160°F, about 35 to 38 minutes.
STEP 4
Meanwhile, on a second rimmed baking sheet, toss squash and apples with remaining 1.5 tablespoons oil, and 0.25 teaspoon each salt and pepper; arrange cut sides down.
STEP 5
Roast on lower rack for 15 minutes. Flip and continue roasting until golden and tender, an additional 5 to 10 minutes.
STEP 6
Sprinkle with parsley and serve with lemony chicken.