tuna noodle casserole

1 medium onion, finely chopped
salt
1.5 tbsp butter
10 oz white mushrooms, trimmed and sliced 1/4 inch thick
2 tsp soy sauce
1/4 cup sherry
1/4 cup flour
2 cups chicken stock
1 cup whole milk
2 tsp lemon juice
1 6-oz can tuna in olive oil, drained
freshly ground black pepper
6 oz curly egg noodles
1.5 cups bread crumbs
1/4 lb cheddar, grated
1 tbsp vegetable oil

Preheat oven to 375 F. Butter a shallow 2 qt baking dish.
Cook onion with a pinch of salt in 1.5 tbsp butter in a 12 inch skillet over moderately low heat, covered, stirring occasionally, until softened, about 5 minutes. Increase heat to moderately high, add mushrooms, and cook, stirring occasionally, until they begin to give off liquid, about 2 minutes. Add soy sauce and continue to cook, stirring, until liquid mushrooms give off has evaporated. Add sherry and boil, stirring occasionally, until evaporated. Remove from heat.
Melt remaining 3 tbsp butter in a 2-3 qt saucepan over moderately low heat. Add flour and cook, whisking, for 3 minutes to make a roux. Add stock in a steady stream, whisking constantly, and bring to a boil, whisking. Whisk in milk, reduce heat, and simmer, whisking frequently, for 5 minutes.
Stir mushroom mixture, lemon juice, and 1/4 tsp salt into sauce. Flake tuna into sauce and stir gently. Season with s & p to taste. Remove from heat.
Cook noodles in a 6 qt pot of boiling salted water until al dente. Drain noodles and return to pot. Add sauce and stir gently to combine. Transfer mixture to baking dish, spreading it 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, 20-30 minutes.