Preheat the oven to 375°F (190°C).
Cut off the tops of the bell peppers and remove the seeds and membranes.
In a large bowl, combine the cooked rice, diced avocado, chopped cilantro, diced red onion, diced tomatoes, lime juice, minced garlic, salt, and pepper. Stir well to combine.
Divide the rice mixture evenly among the bell peppers, pressing it down lightly to fill the peppers.
Place the stuffed peppers in a baking dish and bake for 30-35 minutes, or until the peppers are tender and the filling is heated through.
Serve the stuffed bell peppers hot, garnished with additional cilantro if desired.