Preheat oven to 425°F.
Add 1 teaspoon of salt into boiling water. Cook noodles for 1 minute less than the direction for al dente. Drain and rinse under cold water.
Melt butter in a saucepan over medium-high heat. Add onion and celery. Cook for 3-4 minutes or until soft.
Add flour, cook, keep stirring for 2 minutes or until it smells toasted.
Whisk in the broth, milk, garlic powder, salt, and black pepper. Cook, whisking frequently until thickened (as thick as condensed soup), about 5 minutes. Adjust seasoning with additional salt or a small amount of chicken bouillon if needed.
In a large mixing bowl, add pasta, tuna, cheese, frozen peas, and béchamel sauce. Stir to combine then transfer to a 2 to 2½ qt baking dish.
Sprinkle mixed casserole with Fat Boy Premium All Purpose rub.
Mix panko breadcrumbs with melted butter. Sprinkle on top of casserole.
Bake until bubbly and the cheese is melted for about 18 minutes.