Preheat oven to 425 degrees F and grease a 9x13 inch baking dish with butter. Set aside.
Melt butter in a large pan on medium-high and add onion, celery, garlic, salt, and pepper, cooking for 5-7 minutes until veggies are tender.
Sprinkle on the flour and cook for 1 minute, stirring constantly. Then, slowly whisk in chicken broth and milk, then bring to a simmer.
Add in frozen peas and let the mixture thicken. Once it thickens, add in 1 cup cheddar cheese and stir until melted. Add in tuna and cooked noodles.
Pour into a casserole dish and top with remaining cheddar cheese, panko bread crumbs, and fried onions. Spray over the top with cooking spray. Bake for 15-20 minutes or until the dish is bubbly and golden brown on top.
Notes
*Note - sub any kind of mushrooms (diced) for 1 cup frozen peas if you want mushrooms!