Preheat oven to 375 degrees. Spray a 13x9 baking dish with non-stick cooking spray.
Melt 1 tablespoon butter in a skillet over medium-low heat. Stir in the onion, celery, and garlic, and cook 5 minutes or until tender.
Melt 4 tablespoons butter in a medium saucepan, and whisk in flour until smooth. Gradually whisk in milk and continue cooking 5 minutes, until sauce is smooth and slightly thickened.
Add cheddar cheese. If sauce is too thick, slowly stir in reserved pasta water until sauce is desired consistency. Season with salt and pepper.
Stir in tuna, peas, onion mixture and cooked noodles. Transfer to the baking dish.
Melt remaining 2 tablespoons butter and mix with bread crumbs, and sprinkle over the casserole.
Bake 25 minutes in the preheated oven, or until bubbly and lightly browned.