Preheat oven to 400 degrees and line a sheet tray with parchment paper.
Cook wild rice according to package instructions and set aside.
Wipe any dirt or debris off of the mushrooms. Remove the stem and discard. Using a spoon, scoop the gills out from the cap of the mushroom and discard.
Lightly oil the mushrooms with 1 tablespoon of oil, and sprinkle both sides with salt and pepper. Place the caps facing up, and roast for 10 minutes.
Meanwhile prepare filling: Heat remaining 1 tablespoon oil in a large skillet. Once hot add onion, celery and bell pepper. Cook, stirring occasionally until softened, about 5-7 minutes.
Add spices, dried cranberries, walnuts, veggie stock and balsamic vinegar and bring to a boil. Reduce heat to low and stir in the rice, and let cook for 2-3 minutes.
Remove from heat. If adding goat cheese, fold it now.
Flip the mushrooms over, leaving behind any excess liquid.
Fill the mushrooms. Return to the oven and bake until the mushrooms are tender, about 10-14 more minutes.
Top with some fresh rosemary, and serve hot with your favorite sides.