Note that this recipe includes pre-cooked rice!
Preheat oven to 375°
Heat oil in a large pan over medium heat. Add onions and mushrooms. Cook, stirring frequently 2-3 minutes, until vegetables are softened and fragrant. Add spinach and continue cooking another 1-2 minutes, until spinach is wilted.
Add marinara sauce and pre-cooked rice, stirring gently to combine.
Remove from heat. Fold in Panko bread crumbs and cheese.
Fill each hollowed out bell pepper with rice mixture and place in prepared baking dish. Top with additional cheese, if desired.
Cover and bake for 30 minutes.
Top with extra breadcrumbs and a sprinkling of parsley. Serve hot.