Preheat the oven to 180°C. Keep a deep ovenproof dish or tin and a sheet of aluminium foil handy.
1 Heat the oil and butter in a pot or saucepan and fry the onion until soft. Add the garlic and stir-fry until fragrant.
2 Sprinkle the flour over, stir, then add the chicken stock. Stir until the mixture starts thickening, then mix in the mayonnaise, yoghurt and sweetcorn and season with salt.