With a rack positioned in the middle, preheat the oven to 425º F.
Set the pepper halves on a parchment-lined baking sheet, cut-side down, and roast for 20 minutes, or until slightly tender; remove and set them aside.
While the peppers roast, cook the rice according to the package instructions, but in chicken or vegetable broth if you can.
Heat the oil in a deep skillet. Add the onions and mushrooms and sauté for about 3-4 minutes, then stir in the garlic and sauté for 30 seconds.
Add the ground turkey to the skillet and break it down into crumbles with a spatula or spoon as it cooks. Simmer until the meat is about halfway cooked, then season it with salt and pepper.
Stir in the diced tomatoes and tomato sauce, then bring the mixture to a simmer, stirring occasionally. Once simmering, stir in the cooked rice and the cheese; remove it from the heat.
Spoon the ground turkey mixture into the hollowed-out peppers, packing it as firmly as you can by pressing it down to secure it, then sprinkle the remaining cheese on top.
Transfer the stuffed peppers to the oven for 10-15 minutes, or until the cheese melts and bubbles. For a nice golden crisp on your cheese, turn the broiler on for a minute or two before removing the peppers, but keep a close eye on them so they don’t burn.
Garnish with freshly chopped parsley or basil, and serve!