Wash and chop the runner beans. Place in a saucepan with salted water and bring to a boil.
Cook the beans for about 10 minutes or until al dente. Drain and set aside.
Meanwhile, add the potatoes and cook until tender.
Heat a small skillet with a little butter and sauté the onion until translucent. Add the garlic and cook briefly. Add the runner beans to the pan and stir-fry for a few minutes.
Meanwhile, mash the potatoes with a masher. Add a knob of butter and a splash of milk. Add the bean mixture and grated cheese. Season with salt, pepper and mustard.