Preheat oven to 425 degrees. In a roasting pan or rimmed baking sheet, toss together chicken, squash, onion, and oil; season with salt and pepper. Arrange in a single layer and roast until squash and chicken are cooked through, about 30 minutes.
Transfer chicken to a plate and let cool. Transfer squash and onions to a medium pot and add broth, cumin, and coriander. Bring to a simmer over medium-high. With a potato masher or back of a wooden spoon, mash some vegetables until soup is thick and chunky. Discard skin and bones from chicken; cut meat into small pieces and add to soup. Stir in lemon juice; season to taste with salt and pepper. To serve, top with fresh cilantro, if desired.
Transfer cooled soup to freezer bag or airtight container and freeze, up to 3 months. Thaw, and then reheat. Add garnishes just before serving.