1. Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with foil.
2. Cut around the top of the pumpkin to make a lid. Use a large metal spoon to scoop out the inside membrane and seeds.
3. Place the pumpkin on the prepared baking sheet, and place in the preheated oven. Cook for 1 hour, then lower heat to 325 degrees F (165 degrees C).
4. Meanwhile, place the sausage in a deep pot over medium-high heat. Cook until evenly browned, turning frequently, 10 to 12 minutes. Stir in the carrots, celery, onion, and garlic; cook and stir until translucent and tender, about 5 minutes. If desired, stir in the parsnips, rutabaga, and/or the cabbage; cook until the vegetables are almost ender, about 5 minutes. Add the red and green peppers, broccoli, zucchini, and corn. Pour in the chicken broth, add the rice, and cook 5 minutes more. Stir in the parsley, red pepper flakes, onion soup mix, and Italian seasoning. Season to taste with salt and pepper.
5. Keeping the pumpkin on the baking sheet, spoon the vegetable-sausage mixture into the pumpkin, and replace the lid. Loosely cover the pumpkin with aluminum foil.
6. Cook the pumpkin in the preheated oven until the flesh is tender, about 4 hours. Scoop some of the pumpkin flesh away from the sides to combine with the vegetable mixture. To serve, place the pumpkin on a serving platter, and remove the lid to ladle out the contents.