Preheat the oven to 350F. Spray a 9x13 pan with nonstick spray.
In a large bowl, combine the mixed vegetables, potatoes, onion, mayo, seasoning, celery soup, and cheese.
Spread into the prepared baking dish.
Melt the butter in a small bowl. Crush the crackers and mix with the melted butter.
Top the mixture with the butter/cracker crumb mixture.
Cover and bake for 20 minutes.
Uncover and bake for another 10 minutes or until bubbly and the crackers are golden brown.
Remove and serve.
Notes
If you thaw your frozen veggies first, then make sure to drain any excess liquids.
Lightly spray your casserole dish with cooking oil to make clean up easier afterwards.
When you remove the casserole from the oven, let it rest about 5-8 minutes before serving so the liquids are more fully bound up by the starches and it becomes less "soupy".
When it comes to the cheese topping, then cheddar or Jack work great too, but any cheese that melts nicely works too. I like the mozzarella for that reason especially.