Preheat the oven to 375 degrees F (190 degrees C). Grease a 9×13-inch baking dish.
Bring a large pot of water to a boil. Add egg noodles and 3 teaspoons salt to the boiling water; cook until tender yet firm to the bite, about 6 minutes. Drain noodles. Return to pot and set aside.
Heat 2 tablespoons butter and 1 tablespoon oil in a large skillet over medium-high heat. Add mushrooms and onion and cook until mushrooms are softened and onions are translucent, about 4 minutes. Add remaining butter; stir constantly until melted. Sprinkle flour over vegetable mixture and cook, stirring constantly, for 1 minute. Stir in milk and stock and bring to a simmer, stirring often. Simmer for 5 minutes. Stir in lemon juice, Dijon mustard, 1 teaspoon salt, and pepper.
Add sauce mixture to reserved noodles. Stir in 1 cup Cheddar cheese, peas, and tuna. Pour mixture into the prepared baking dish and sprinkle remaining Cheddar cheese over the top.
Stir together bread crumbs, remaining 2 teaspoons oil, remaining 1/4 teaspoon salt, and paprika in a small bowl. Sprinkle bread crumb mixture evenly over the casserole.
Bake in the preheated oven until golden and bubbly, 25 to 30 minutes. Sprinkle with parsley and serve.
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