Preheat oven to 375°F.
Remove sausage from casings and crumble into small bits. Place Italian sausage in a large skillet over medium-high heat, allow to brown. Add 2 cups of diced portobello mushrooms, mustard, sage and parsley. Cook until mushrooms are tender. Season with salt and pepper to taste. Place meat and mushroom mixture into well greased skillet. In a separate bowl, combine 4 cups baked sweet potatoes with 3 tbsp brown sugar. Spread potato mixture over ground meat. Sprinkle blue cheese, panko bread crumbs, and chopped toasted walnuts. Place mixture into preheated oven and bake until golden brown, about 15 minutes.