Preheat oven to 375˚F.
Grease a 9x13 baking dish with cooking spray and set aside.
Cook the noodles according to the directions on the package.
Drain noodles and return drained noodles back to the pot; set aside.
Melt the butter in a large saucepan or large deep skillet set over medium heat.
To the melted butter, add celery, peppers and onions; cook for 8 to 10 minutes or until vegetables are tender.
Stir in the garlic and cook for 20 seconds.
Stir in flour, mustard, salt and pepper.
Add milk; cook and stir until bubbly and thickened. Sauce will be thick.
Pour the sauce over the noodles.
Add flaked tuna and cheese to the noodles.
Gently fold and mix until everything is thoroughly combined.
Transfer tuna mixture to the baking dish; set aside.
In a mixing bowl, combine crushed ritz crackers and butter; sprinkle over the casserole.
Bake uncovered for 25 to 30 minutes, or until heated through.
Remove from oven and let stand for 5 to 8 minutes before serving.