1. Preheat oven to 375ºF. Warm oil in a medium skillet over medium heat. Add onions and bell pepper and cook, stirring, until softened, 5 to 7 minutes. Stir in chicken and season with salt. Cook, stirring, until heated through.
2. Pile chicken mixture onto bottom halves of rolls. Top each portion with a slice of cheese and top halves of rolls. Place sandwiches on a baking sheet and bake until cheese has melted, about 5 minutes. Serve immediately.