In a large skillet on top of stove, saute onion in butter.  Add Broccoli, mix.  Add soup (undiluted), stir until combined.  Add about 1/2 jar (or more, if you like it Cheesier) of the Cheese Whiz, stir until all combined.  Add cooked rice, combine.  Put the mixture in an oven-proof casserole dish.  Bake for 30 minutes at 350 degrees.