Preheat oven to 350°F (175°C).

Drain the canned mixed vegetables (Veg-All) and water chestnuts.

Chop the onion.

In a large bowl, combine the drained mixed vegetables, chopped onion, mayonnaise, and shredded cheese.

Season with salt and pepper to taste.

Pour the mixture into a greased casserole dish.

In a frying pan, melt butter over medium heat.

Crumble Ritz crackers into the melted butter and stir to coat.

Sprinkle the cracker mixture evenly over the top of the casserole.

Bake for 25 minutes, or until the topping is golden brown and the casserole is bubbly.

Optional: For a more moist casserole, add a can of mushroom soup to the vegetable mixture before baking.