The first time I made this for my boys, I cooked chicken on the side thinking they would never eat so many vegetables in one sitting - they ate every last bite and never even touched the chicken! This recipe has been a dinner mainstay every since.
Slice tops off peppers, remove seeds. When the water is boiling, add 1 tbsp salt then add the peppers and cook until almost soft - 3 to 4 minutes. Using tongs, remove the peppers from the water and place in a colander to cool, cut sides down. Reserve the cooking water.
In a large skillet, over medium heat
Melt 2 tbsp butter. Add half the onions and saute until soft, about 5 minutes. Add the uncooked rice and continue cooking, stirring frequently, until the rice begins to turn golden, about 10 minutes. Add 3 cups of the peppers' cooking water and bring to a boil. Reduce the heat to low, cover, and cook until the liquid is completely absorbed, 12-15 minutes. Remove from heat and place a paper towel between the pot and the lid to absorb any remaining moisture.
In a separate skillet over medium heat
Melt 2 tbsp butter. Add the remaining onions, mushrooms, celery, corn, spinach, ginger, garlic, and cayenne, saute until vegetables are tender, about 10 minutes. Add salt to taste.
Combine contents of both skillets
In a large bowl, mix all the vegetables and the rice mixture and half the Parmesan cheese. Add fresh pepper to taste.
Fill and bake
Place the peppers in a glass 9" square baking dish. Fill each pepper with the rice mixture. Spread the extra rice mixture around the peppers. Top with the remaining Parmesan cheese. Bake until heated through, about 20 minutes.