Preheat oven to 375 degrees. Peel pumpkin and squash, remove seeds, and cut into small chunks. Toss pumpkin, squash, and leeks with olive oil, salt, pepper, and garlic. Spread evenly on baking tray and roast for 25 minutes, until tender. When tender, remove from oven and transfer to large saucepan. Add 3 cups of chicken stock and mix with hand blender, breaking down squash, pumpkin, and leeks. Add remaining broth, brown sugar, and nutmeg and blend until the desired consistency is reached. Cook over medium heat for 30 minutes, adding cream and season to taste.