Preheat oven to 400°F Line baking pan with parchment paper.
Cut each squash lengthwise in half (from tip to stem) then scoop out seeds and strings. Make a small slice on bottom of each half so it sits flat. With pastry brush, coat squash cavities with oil. Sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper. Place halves, cut side down, in prepared pan and roast until tender but still firm, about 30 minutes.
In a large skillet, heat oil over medium heat. Add onions and cook, stirring often, until translucent. Add seitan and cook 2 minutes. Stir in garlic, celery, bell pepper, mushrooms, sage, rosemary, salt and pepper and cook, stirring occasionally for 10 minutes. Stir in parsley and remove from heat.
In large bowl, combine cooked vegetables, cornbread, tamari, and vinegar and mix well. Stir in enough broth so that stuffing is moist enough to hold together when pressed but not soggy. Adjust seasoning to taste with tamari, vinegar, salt and pepper.
Remove squash from oven and reduce heat to 300°F Carefully turn squash; fill each cavity with 1/2 cup stuffing. Return squash to oven and bake until topping begins to brown, about 25 minutes. Serve hot.