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
SERVE in the Marine Life Casserole from Vagabond House