For the casserole:
Coarse salt and ground pepper
10 oz. green beans, stem ends trimmed, cut into 1 1/2-inch lengths
1/2 lb. gemelli or other short pasta
2 cans (5 oz. each) solid white albacore tuna packed in water, drained
5 slices white sandwich bread, crusts removed, torn into large pieces
2 T. unsalted butter, melted
Béchamel Sauce (recipe below)
For the sauce:
3 T. unsalted butter
1/4 cup finely shopped onion
1/4 cup all-purpose flour
1/2 tsp. coarse salt
4 cups whole milk
Start with the sauce:
In a medium saucepan, melt butter over medium heat. Add onion and cook until softened, about 6 minutes. Add flour and salt; cook stirring frequently, until mixture is pale golden, has a slightly nutty aroma, and is the texture of cooked oatmeal, about 5 minutes.
Whisking constantly, pour in 2 cups milk; add 2 more cups milk and whisk until smooth. Cook mixture, stirring constantly along the bottom of pan, until boiling, about 7 minutes.
Reduce heat to low. Simmer gently, stirring occasionally, until sauce thickens and does not feel grainy when a very small amount is rubbed between two fingers, about 10 minutes. (To store, press a sheet of plastic wrap on the surface and let sauce cool. Transfer to a resealable container and refrigerate, up to 5 days.)
For the casserole:
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 350º. In a large bowl, combine pasta, green beans, and béchamel; 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 to a 2-quart baking dish and top with breadcrumbs. Bake until sauce is bubbling and breadcrumbs are golden brown, 25 to 30 minutes.