Bring a pot of water to a boil and cook the egg noodles to al dente according to the package instructions. Drain and set aside.
Meanwhile, melt 4 tablespoons of the butter in a large skillet over medium heat. Add the onion and cook until starting to soften, 3 to 4 minutes. Add the mushrooms and cook, stirring, for a couple of minutes. Sprinkle the flour evenly over the mixture and stir so that the flour coats the onions and mushrooms thoroughly. Cook, stirring, for another minute. Add the milk and sherry and whisk to combine. Cook the sauce until it's nice and thick, 3 to 4 minutes. Sprinkle in salt and pepper to taste and stir. Adjust the seasoning to make sure the sauce is adequately salted.
Add the tuna and stir it into the sauce. Then stir in the bell pepper and 1 tablespoon of the parsley. Stir in the noodles until they're coated. Butter a 9-by-13-inch baking dish and transfer the noodle mixture to the dish.
Melt the remaining 4 tablespoons butter in a bowl in the microwave; stir it around with the breadcrumbs and remaining tablespoon of parsley.
Top the casserole with the breadcrumbs and bake until golden, 25 to 30 minutes. Serve piping hot with a green salad and warm crusty bread.
To make ahead: Assemble the casserole and refrigerate, unbaked, up to 24 hours before baking. Allow an extra 10 to 15 minutes of baking time if you put it in the oven cold.