Preheat the oven to 375 degrees F and grease a 2-quart baking dish.
In a large heavy-bottomed skillet, melt 1 tablespoon butter over medium-high heat. Saute the onions until translucent and fragrant; season with salt and pepper. Add the mushrooms and cook, stirring occasionally for about 2 minutes. Stir in the sherry and soy sauce and cook until the majority of the liquid has evaporated. Remove from the heat and set aside.
Melt the remaining 4 tablespoons of butter, in large heavy-bottomed saucepan over medium-low heat. Whisk in the flour and cook for about 2 to 3 minutes. Slowly add the chicken broth, whisking constantly. Bring to a boil, then whisk in the milk. Reduce the heat to a simmer and continue cooking for an additional 5 minutes.
Add the mushroom mixture to the white sauce and stir in the lemon juice. Season the sauce with salt and pepper, to taste. Add the drained tuna and the peas; stir just to combine. Check seasoning and add more salt and pepper, if needed. Remove from heat.
Cook the egg noodles in a large pot of salted boiling water according to package directions just until al dente. Drain and return it to the saucepan. Add the sauce, stirring gently to coat the noodles. Transfer the mixture to the prepared baking dish.
In a mixing bowl, toss together the grated cheese and the bread crumbs. Drizzle with the 1 tablespoon of melted butter and toss. Sprinkle evenly over the casserole. Bake until the topping is crisp and the sauce is bubbling.