Preheat oven to 220 degrees C. Grease a 32cm x 45cm rimmed baking tray with cooking spray.
Whisk 2 tablespoons olive oil, mustard and apple cider in a bowl.
Sprinkle chicken breasts with rosemary, 1 teaspoon salt and 1/4 teaspoon pepper. Place on one side of the baking tray; brush with mustard mixture. Place pumpkin on the other side of the baking tray; drizzle with remaining 2 tablespoons olive oil, 1/2 teaspoon salt and 1/4 teaspoon pepper; toss until well coated.
Roast in the preheated oven until pumpkin is tender and chicken is golden, 35 to 40 minutes. Baste chicken with mustard mixture several times during roasting.