For the bean and bell pepper salad, soak the beans overnight with three times the amount of water, then boil separately for about 40-60 minutes until soft, depending on size, and drain. Cook cleaned runner beans in salted water until al dente. Remove stem and seeds from peppers. Cut peppers and onion into fine rings or strips.
For the marinade, mix all ingredients well with a whisk. Rub a bowl with garlic and mix the beans, still warm, with the green beans, peppers and onions. Pour over the marinade, mix through and let sit for 10 minutes. Garnish the bean and bell pepper salad with lettuce leaves.