Preheat oven to 325 degrees.
Spray a 8"x12"x3" casserole with cooking spray. Set aside.
Put green beans in a small pot with a cup of water and 1/4 teaspoon salt. Bring to a boil, reduce heat, and cook until tender, 3-5 minutes. Drain and set aside.
Put corn in a small pot with a cup of water and 1/4 teaspoon salt. Bring to a boil, reduce heat, and cook until tender, 2-3 minutes. Drain and set aside.
In a large bowl, mix together the onion, celery, bell pepper, cheese, corn, green beans, cream of celery soup, sour cream, salt and pepper.
Pour into prepared casserole.
Melt butter and mix with Ritz crackers. Spread evenly on top of the vegetable mixture.
Bake for 60 minutes to 75 minutes, until bubbly.
Cool slightly before serving.