Preheat the oven to 400˚. Bring a large pot of salted water to a boil. Add the egg noodles and cook as the label directs for al dente. Drain and set aside.
Meanwhile, melt 4 tablespoons 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, until softened. Sprinkle the flour evenly over the mixture and stir so that the flour coats the onion and mushrooms thoroughly. Cook, stirring, until toasted, about 1 more minute. Add the milk and sherry and whisk to combine. Cook the sauce, whisking, until it’s nice and thick, 3 to 4 minutes. Season with salt and pepper and stir. Adjust the seasoning to make sure the sauce is adequately salted.
Add the tuna to the sauce and stir, 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; combine it with the panko and remaining 1 tablespoon parsley.
Top the casserole with the panko mixture and bake until golden, 25 to 30 minutes.