Cook noodles according to package directions. Drain, rinse (to avoid sticking), and set aside.
Meanwhile, combine bread crumbs and 2 tablespoons melted butter; set aside.
Preheat oven to 375°F.
For sauce, melt 4 tablespoons butter in a medium saucepan (at least 3 quart-size) over medium heat. Sauté celery and onion until tender. Sprinkle flour and dry mustard on top of veggies, let sit for 30 seconds, and then stir into mixture. Increase heat to medium-high and slowly add milk and broth while stirring constantly. Continue to cook and stir constantly until mixture is slightly thickened and bubbly.
Grease a 9x13” casserole dish with butter. Combine noodles, tuna, fresh or frozen veggies, and sauce in the casserole dish. Sprinkle with bread crumb mixture. (At this point, you could refrigerate the casserole until you are ready to bake it for dinner.)
Bake, uncovered, for 30 minutes or until bubbly and just starting to brown on top. Enjoy!