In a 12-inch cast-iron skillet, heat 1 tablespoon oil over medium-high heat. Add onion and sweet peppers; cook, stirring occasionally, until vegetables are lightly browned, about 8 minutes. Remove from skillet.
In skillet, heat remaining 3 tablespoons oil over medium-high heat. Add potatoes; cook, stirring occasionally, until tender and golden brown, about 10 minutes.
In a small bowl, whisk together ketchup, ¼ cup water, cilantro, mustard, chili powder, salt, and black pepper. Stir chicken, vegetables, and ketchup mixture into potatoes. Cook, stirring occasionally, until heated through.
Recipe by Southern Cast Iron at https://southerncastiron.com/chicken-and-pepper-hash/