Cook the Pasta : Fill a large pot with salted water and bring it to a boil. Add the elbow macaroni and cook according to the package directions until al dente. Drain and set aside.

Sauté the Vegetables : Warm the vegetable oil in a large skillet over medium heat. Add the onions, celery, and leeks, and sauté for 5–7 minutes, until softened but not browned.

Make the Sauce : In the same skillet, melt the butter over medium heat. Whisk in the flour and cook for 1–2 minutes to create a smooth roux. Gradually whisk in the chicken stock, followed by the milk and heavy cream. Cook, stirring constantly, until the sauce thickens (about 5 minutes).

Assemble the Casserole : Add the flaked tuna, sautéed vegetables, dry sherry, frozen peas, and Parmesan cheese to the sauce. Stir to combine. Fold the cooked macaroni into the tuna mixture. Transfer everything to a greased casserole dish.

Add the Topping : Sprinkle the fresh breadcrumbs evenly over the casserole.

Bake : Preheat your oven to 375°F (190°C). Bake the casserole for 25–30 minutes, or until the top is golden brown and the filling is bubbling.

Serve : Take the casserole out of the oven and allow it to sit for 5 minutes before serving. Serve while warm and enjoy!