Wash and peel the potatoes and cut into thick slices. Wash the runner beans and cut into 2.5 cm long pieces. Heat the butter and oil in a pan. Add the chillies, garam masala, cumin and turmeric. Squeeze in the garlic. Stir-fry for 30 seconds. Add the potatoes to the pan, salt and stir-fry until they are coated with the fat. Add the runner beans and a little water and cook until firm to the bite, about 30 minutes.