Heat the sunflower oil in a large pan and stir-fry the onion, bell pepper, and chicken thighs over a high heat for 3-4 minutes until well browned. Stir in the chiles, garlic, tomatoes, and corn and cook for another 2-3 minutes until the chicken is cooked through.
Stir in the taco sauce and green onions, and set aside to cool a little.
Warm one of the tortillas in a hot, dry frying pan until soft and flexible. Spoon a quarter of the cooled chicken mixture into the center of a tortilla and fold over the edges to form a neat package; pin in place with wooden toothpicks and repeat with the remaining tortillas.
Heat 1/2 inch vegetable oil in a large frying pan and cook chimichangas for 2-3 minutes on each side until crisp and golden brown. Drain on paper towels and serve hot with shredded lettuce, sour cream and guacamole.