I most frequently make this with sweet potatoes and portobello mushrooms because they are my family's favorites, and that is how I wrote this recipe.  However it is also wonderful using acorn squash, baking potato, eggplant, sweet pepper, heck hot pepper even.  Once you make the stuffing and walnut breadcrumb topping, you can personalize the meal by putting it into everyone's respective favorite veggie vessel, adjusting time in the oven based on it.