Preheat the oven to 375 degrees. Cut the tops off the bell peppers and remove the seeds and membranes. Place the peppers upright in a baking dish.
Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for 3 minutes until it softens.
Add the minced garlic and chopped mushrooms to the skillet. Cook for 5 minutes, stirring often, until the mushrooms release their liquid and start to brown.
Add the ground turkey to the skillet along with the Italian seasoning, salt, and black pepper. Break up the turkey with a spoon and cook for 8 minutes until it is no longer pink.
Spoon the turkey and mushroom mixture evenly into the prepared bell peppers. Top each pepper with the shredded mozzarella cheese.
Bake the peppers for 25 minutes until the cheese melts and the peppers are tender. Remove from the oven and sprinkle with the chopped parsley before serving.