If the words tuna noodle casserole conjure up school-cafeteria nightmares, give our version a try. The original formula graduates to gourmet status with the inclusion of Dijon mustard, quality cheddar, and the finest oil-packed tuna you can find.
Heat the oven to 350°F. Butter an 8-by-8-inch glass baking dish, and bring a large pot of salted water to a boil.
Heat olive oil and butter in a medium saucepan over medium heat. When oil is hot and butter foams, add celery and cook, stirring occasionally, until soft but not browned, about 5 minutes. Stir in scallions and jalapeño, and cook for 3 minutes more.
Add Old Bay Seasoning and flour to vegetable mixture, stirring until flour is well incorporated. Cook for 1 minute, then slowly pour in milk, stirring well to make sure no lumps of flour are left. Bring mixture to a boil, reduce heat to medium low, and simmer, stirring occasionally, until mixture thickens slightly, about 5 minutes. Remove from heat and adjust seasoning with salt and freshly ground black pepper. Stir in sour cream and mustard, and set aside.
Once the water is boiling, add egg noodles and cook until they are slightly tender and still white in the middle, about 3 minutes. Drain.
Stir cheddar and egg noodles into vegetable-and-milk mixture. Flake tuna into mixture, and stir gently. Pour into the prepared baking dish.
For the topping:
Combine parsley, butter, cheddar, and saltine crumbs in a mixing bowl, using your hands to mix well until all butter is incorporated. Sprinkle mixture evenly over casserole. Bake for 20 to 25 minutes or until crackers are golden and casserole is bubbling.