Preheat the oven to 350 degrees F (175 degrees C).

Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add frozen vegetables, cover, and steam until tender, 4 to 6 minutes. Transfer vegetables to a baking dish.

Mix mayonnaise, sour cream, onion, and Cheddar cheese in a bowl; pour over vegetables. Stir crackers and melted butter in another bowl and sprinkle over vegetables.

Bake in the preheated oven until golden brown and bubbly, about 30 minutes.