Heat the oven to 375 °F. Cut the tops off the bell peppers, then scoop out the seeds and white membranes. Stand the peppers upright in a 9x13-inch baking dish; if one wobbles, slice a thin sliver off its bottom so it sits flat.
Heat the olive oil in a large skillet over medium heat. Add the onion and cook, stirring, until soft, about 4 minutes. Add the turkey and cook, breaking it apart with a spoon, until no longer pink, 6 to 7 minutes.
Stir in the chili powder, smoked paprika, salt and black pepper and cook for 30 seconds. Take the skillet off the heat and stir in the rice, 1 cup of the drained tomatoes and 1/2 cup of the mozzarella.
Spoon the filling into the peppers, mounding it slightly. Pour the reserved tomato juice and any remaining tomatoes into the bottom of the baking dish.
Cover the dish tightly with foil and bake for 35 minutes. Remove the foil, sprinkle the remaining 1/2 cup mozzarella over the tops, and bake 10 to 15 minutes more, until the cheese is melted and the peppers are tender.
Let the peppers rest for 5 minutes, then spoon some of the tomato juices from the pan over each one before serving.
