Preheat the oven to 350°F. Grease a 2-quart baking dish.
Cook the vegetables according to the package directions.
In a large bowl, combine 1 cup of cheese, condensed soup, sour cream, milk, and onion powder. Stir well to combine. Add the cooked vegetables, toss to combine, and transfer to the prepared dish.
In a small bowl, melt the butter. Add the crushed crackers and remaining cheddar cheese and stir to combine. Sprinkle over the casserole.
Bake for 30 to 35 minutes until golden brown.
Notes
California blend includes broccoli, cauliflower, and carrots.Leftovers will keep in an airtight container in the refrigerator for 3 days.