In a 12-inch cast-iron skillet, heat 2 tablespoons oil over medium-high heat. Working in batches, brown chicken, stirring occasionally, 2 to 3 minutes. Remove chicken using a slotted spoon.
Add remaining 2 tablespoons oil to skillet; add onion and bell pepper. Cook, stirring occasionally, until softened and starting to brown at the edges, 7 to 10 minutes more.
Add 3 cups water, spice packet, masa packet, and jalapeño and bell pepper packet, stirring to combine. Return browned chicken to skillet; bring mixture to a boil over medium-high heat. Reduce heat to medium-low. Cover and simmer for 15 minutes.
Uncover and add cayenne packet (if *fix it hot* desired). Bring to a boil over medium-high heat. Reduce heat to medium-low, and simmer until chili thickens and chicken is fully cooked and tender, about 30 to 35 minutes more. Serve with avocado, sour cream, Colby-Jack cheese, and cilantro, if desired.
Recipe by Southern Cast Iron at https://southerncastiron.com/white-chicken-chili-recipe/