Preheat oven to 375° F.
In a large mixing bowl, combine noodles, tuna, shredded cheese, and diced pimentos. Set aside.
Melt 3 Tbsp. butter in a small saucepan. Add celery and onions and cook until tender.
Stir in flour, dried mustard, and salt.
Add milk and chicken broth all at once; cook and stir until thickened and bubbly.
Pour sauce over noodle mixture and mix together.
Transfer mixture to a 1 1/2 quart casserole dish.
Top with a mixture of bread crumbs and 2 Tablespoons melted butter.
Sprinkle with Italian seasoning.
Bake for 20-25 minutes.