Boil the beans in salted water, drain and let them cool. Cut the tomatoes and place them on a plate. Add the onion, tuna and green beans. Season the salad with oil, vinegar, pepper and salt.
In a large pan, melt the butter over medium-low heat. Add the gnocchi and sauté for 5 minutes. Transfer the gnocchi into the plates, add the slices of the hard-boiled eggs and serve.