Cut cauliflower florets from stem. Boil in water until tender, but still slightly crisp. Drain and set aside.
To make the cheese sauce, saute onions in 1/4 cup butter. Set aside. In a small bowl, combine flour, Cajun seasoning, and half and half. Add this to saucepan containing onions. Heat over medium heat until mixture begins to thicken, stirring frequently. Remove from heat. Blend in Parmesan cheese. Set cheese sauce aside.
Place cauliflower in lightly greased 1-1/2 quart casserole dish. Pour sauce over cauliflower and set aside.
Combine 2 tablespoons butter and bread crumbs. Sprinkle bread crumb mixture around edges of casserole. Top with cheddar cheese and bake at 350 degrees F for 25 minutes.