Preheat oven to 350 degrees F and grease a 9x13 inch baking dish with cooking spray.
Melt 2 tablespoons butter over medium heat in a large sauté pan.
Add in onion, bell peppers, garlic, salt, pepper, and paprika.
Cook until the veggies are crisp-tender, about 5-10 minutes. Set aside.
In a large bowl, mix together cream of mushroom soup, sour cream, and cream cheese.
Mix in pepper mixture and chicken.
Taste and re-season, if necessary.
Pour into prepared pan, then sprinkle with shredded cheese.
Melt the remaining 4 tablespoons of butter and mix with the crushed ritz crackers.
Sprinkle over the chicken mixture.
Bake in preheated oven for 30-35 minutes or until the casserole is hot and the cheese has melted.