Boil the noodles. Heat a large pot of salted water. Cook the egg noodles until just shy of al dente. Drain and set aside. Toss with a splash of olive oil to prevent sticking.

Prep your baking dish. Grease a 9x13-inch casserole dish or similar. Preheat oven to 375°F (190°C) with a rack in the center.

Sauté aromatics. In a large skillet, melt 2 tablespoons butter over medium heat. Add onion and celery with a pinch of salt. Cook 4–5 minutes until soft. Add garlic (and mushrooms if using) and cook 1–2 minutes more.

Make the sauce. Add 3 tablespoons butter to the skillet. Once melted, sprinkle in the flour and whisk, cooking 1–2 minutes until it smells toasty. Slowly whisk in milk, then broth, until smooth. Simmer 3–5 minutes, whisking, until slightly thick and creamy.

Season and add cheese. Stir in Dijon, a pinch of black pepper, and 1/2 teaspoon kosher salt. Add 1 cup shredded cheese and whisk until melted and smooth. Taste and adjust salt and pepper. The sauce should be well seasoned.

Fold everything together. Off heat, add peas, drained tuna (flaked with a fork), lemon zest, and a teaspoon of lemon juice. Gently fold in the cooked noodles and parsley. Avoid over-stirring so the tuna stays flaky.

Assemble. Transfer mixture to the baking dish. Sprinkle the remaining 1/2 cup cheese on top.

Make the topping. In a small bowl, mix panko with 1 tablespoon olive oil, a pinch of salt, and a few grinds of pepper. For extra flavor, add a little paprika or grated Parmesan. Sprinkle evenly over the casserole.

Bake. Cook 18–22 minutes, until the edges bubble and the topping is golden. If needed, broil for 1–2 minutes to brown the top. Let rest 5–10 minutes before serving.

Finish and serve. Squeeze another teaspoon of lemon juice over the top if you like, and garnish with extra parsley. Serve warm.