In a large bowl, mix together the shredded chicken, rice, enchilada sauce, 1/2 cup shredded cheddar cheese, red onion, and 3 tablespoons cilantro. Set aside.
Fill the slow cooker with 1/3 cup water.
Rinse bell peppers, and then cut around the tops of the bell peppers removing the seeds and stem. Carefully begin filling the bell peppers with the chicken-rice mixture. Place stuffed bell peppers in the slow cooker. Top with remaining 1/4 cup shredded cheddar cheese. Cook on high for 3 hours.
Serve immediately and top with diced green onions, cilantro, and sour cream.