Preheat oven to 400* F. Chop broccoli into florets and cauliflower into more finely chopped pieces. Toss with olive oil and some salt and pepper in a 3 quart baking dish. Place in the oven for 20 minutes, stirring halfway.
Meanwhile, rinse and drain cannellini beans.
In a blender, combine beans, milk, 1 3/4 cups grated cheddar cheese, mustard, onion powder, and salt. Blend until well combined.
After 20 minutes, remove broccoli and cauliflower from the oven and mix the blender sauce evenly into the veggies. Top with remaining 1/4 cup cheese.
Place casserole back into the oven for 25-30 more minutes, until veggies are softened to liking.