Preheat the oven to 400°F (200°C).
Heat a large skillet over medium heat and add the ground chicken and finely diced onion; cook, breaking the meat up with a spatula, until the chicken is cooked through, about 6–8 minutes. Drain any excess liquid from the pan if needed.
Stir in the Italian seasoning, garlic powder, kosher salt, and black pepper until evenly combined.
Add the fresh spinach, cooked rice, and fire-roasted diced tomatoes (with juices) to the skillet; cook, stirring occasionally, until the spinach is wilted and the mixture is heated through.
Meanwhile, cut each bell pepper in half lengthwise and remove the ribs and seeds; arrange the pepper halves cut side up in a 9×13-inch baking dish.
Spoon a heaping scoop of the meat and rice mixture into each pepper half, distributing it evenly. Sprinkle shredded mozzarella cheese over each stuffed pepper.
Pour about 1/2 cup of water into the bottom of the baking dish, then bake in the preheated oven until the peppers are tender and the cheese is melted, about 20–30 minutes.
Garnish with chopped fresh parsley and serve immediately.