Preheat oven to 350°. Spray a 9x13 baking dish with non-stick cooking spray. Set aside while you prepare the casserole.
In a large pot over medium-high heat add 6 cups of water. Bring to a rolling boil. Add a pinch of salt to the water. Cook and drain 3 cups of pasta shells per package directions, until al dente.
In a large bowl, mix together pasta, drained and flaked tuna, sweet peas, cream of celery soup, milk, sour cream, Old Bay Seasoning, salt, and pepper. Mix until well combined.
Place mixture in prepared casserole dish. Spread evenly. In a small bowl, combine plain breadcrumbs and melted butter. Mix until crumbly.
Cover the top of the pasta mixture with 1 cup of grated mild cheddar cheese. Sprinkle buttery breadcrumbs on top of the cheese.
Bake, uncovered, in a preheated oven for 45 minutes, until cheese is melted and casserole is bubbling.
Allow to rest 10 minutes before serving.