Heat the oil in a saucepan, add the whole chicken breast and the onion and fry over a low heat for about 5 minutes to soften but not colour. Add the stock, milk, potato and sweetcorn and bring to the boil. Reduce the heat, cover and simmer for 10 minutes until the potato and chicken are cooked through.
Remove the chicken from the pan, place on a board and shred or chop. Blend the soup with a hand-held blender or in a food processor until almost smooth. Season with salt and pepper, return the chicken to the pan and reheat.