Preheat the oven to 375°F (190°C).
In a medium saucepan, bring the chicken broth to a boil. Add the rice, reduce heat to low, cover, and simmer for 15 minutes or until rice is tender and liquid is absorbed. Remove from heat and let cool slightly.
While the rice cooks, heat olive oil in a skillet over medium heat. Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and cook for an additional 1 minute until fragrant.
Stir in the chopped spinach and cook until wilted, about 2 minutes. Remove from heat.
In a large bowl, combine the cooked rice, sautéed onion, garlic, spinach, mozzarella cheese, Parmesan cheese, Italian seasoning, salt, and black pepper. Mix well.
Using a sharp knife, carefully cut a pocket into the side of each chicken breast, being careful not to cut all the way through.
Stuff each chicken breast evenly with the rice mixture and secure the opening with toothpicks if necessary.
Place the stuffed chicken breasts in a baking dish and bake for 30-35 minutes or until the chicken is cooked through and juices run clear (internal temperature should reach 165°F or 74°C).
Remove from the oven and let rest for 5 minutes. Remove toothpicks before serving.
Garnish with fresh chopped parsley and serve warm.