Put oven rack in middle position and preheat oven to 375°F. Butter a shallow 2-quart baking dish.

Cook onion in 1 1/2 tablespoons butter with a pinch of salt in a 12-inch heavy skillet over moderately low heat, covered, stirring occasionally, until softened, about 5 minutes. Increase heat to moderately high and add mushrooms, then sauté, stirring occasionally, until mushrooms begin to give off liquid, about 2 minutes. Add soy sauce and continue to sauté mushrooms, stirring, until liquid mushrooms give off is evaporated. Add Sherry and boil, stirring occasionally, until evaporated. Remove from heat.

Melt remaining 3 tablespoons butter in a 2- to 3-quart heavy saucepan over moderately low heat and whisk in flour, then cook roux, whisking, 3 minutes. Add broth in a stream, whisking, and bring to a boil, whisking. Whisk in milk and simmer sauce, whisking occasionally, 5 minutes. Stir in mushroom mixture, lemon juice, and salt. Flake tuna into sauce and stir gently. Season sauce with salt and pepper.

Cook noodles in a 5- to 6-quart pot of boiling salted water until al dente. Drain noodles in a colander and return to pot. Add sauce and stir gently to combine. Transfer mixture to baking dish, spreading evenly.

Toss together bread crumbs and cheese in a bowl. Drizzle with oil and toss again, then sprinkle evenly over casserole. Bake until topping is crisp and sauce is bubbling, 20 to 30 minutes.