Preheat oven to 350 degrees F (175 degrees C). Grease a 2-quart baking dish.
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until cooked through but firm to the bite, 8 minutes. Drain and return macaroni to pot.
Stir 1/4 cup butter, 1/4 cup milk, and powdered cheese from envelope into the cooked macaroni. Stir cream of mushroom soup, tuna, and 1/2 cup milk into the macaroni and cheese. Pour macaroni mixture into the prepared baking dish; top with Cheddar cheese and parsley.
Mix bread crumbs, melted butter, and dill in a small bowl; sprinkle over Cheddar cheese layer.
Bake in the preheated oven until cheese is melted and bread crumb topping is lightly browned, about 20 minutes.