Mix onion, cream of mushroom soup and Velveeta in a crockpot on high until melted. Add broccoli and cauliflower until hot. Add cooked shells right before serving.
Put half of broccoli-cauliflower on bottom of greased pan. ... broccoli/cauliflower, sprinkle half of crackers on cheese. Repeat. Bake at 300 degrees for 1 hour.