This classic dish with tuna and green beans cooks up nicely in half an hour.
In a large saucepan of well-salted boiling water, cook green beans until almost tender, about 5 minutes. Using a slotted spoon, transfer green beans to a colander (reserving water). Rinse with cold water until cool; drain. Return water in pan to a boil; add pasta and cook 2 to 3 minutes short of al dente; drain and rinse pasta.
Preheat oven to 350 degrees. In a large bowl, combine pasta, green beans, and bechamel; season with salt and pepper. Gently fold in tuna.
In a food processor, pulse bread until coarse crumbs form. Transfer to a bowl and stir in butter. Transfer tuna mixture to a 2-quart baking dish and top with breadcrumbs. Bake until sauce is bubbling and breadcrumbs are golden brown, 25 to 30 minutes.