Preheat the oven to 375°F. Grease a 9x13 baking dish.
In a large bowl, combine the drained Veg-All, diced onion, diced celery, sliced water chestnuts, cream of chicken soup, salt, and pepper. Stir until everything is evenly combined.
Fold in 1 ½ cups of the shredded cheddar and stir to incorporate.
Transfer the mixture to the prepared baking dish and spread into an even layer.
Sprinkle the remaining ½ cup of cheddar over the top.
Cover evenly with the crushed Ritz crackers. Lay the sliced butter pieces over the cracker topping.
Bake at 375°F for 30 minutes until the topping is golden and the casserole is bubbling around the edges.
Top with sliced green onions and serve directly from the dish.