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.
