Wash the chicken breast and pat dry. Fry over medium-high heat in a pan with oil for 4 minutes per side until golden brown. Season with salt and pepper and transfer to a baking dish.
Put the mandarin oranges with their juice, brown sugar, soy sauce, tomato paste, vinegar, finely chopped onion, and cornstarch in a blender and puree until smooth.
Pour the blended mandarin sauce into a pan and bring to a boil over medium-high heat. Simmer for 1 to 2 minutes and season with salt and pepper.
Pour the sauce over the chicken in the baking dish. Bake for 40 minutes at 200°C (392°F) until the chicken reaches an internal temperature of 74°C (165°F).