Preheat oven to 350°F.
Cook rice in saucepan for 20 minutes, or until water has been absorbed. You want the rice to be al denté because it will finish cooking in in the oven. Set aside and let cool.
Cut the top off of the peppers (do not discard) and remove core. Rub peppers with olive oil, salt and pepper.
Place peppers in a baking dish standing upside down and bake for 15 minutes.
Preheat sauté pan on medium heat and add oil. Once oil is hot, add mushrooms and garlic, then sauté for 2 minutes. Add mirin and sauté for 1 more minute. Remove from heat and let cool.
Combine remaining ingredients (including ground pork and par-cooked rice) and mushrooms in a mixing bowl.
Stuff cooled peppers with pork filling. Cover stuffed pepper with the pepper top and bake for 1 hour, or until meat thermometer inserted into pepper reads 165°F. Let cool 5 minutes before enjoying.