pulse thawed cauliflower in food processor once or twice just breaking it up.. large chop!!!
put 1" of water in the bottom of a Dutch oven. bring to a boil and turn down to low. cover and cook 5 min. should be crisp but tender. drain in colander for several minutes.
in a LG saucepan saute onions in butter. add mustard, salt, pepper, & flour. after the Roux is made add milk slowly. should thicken to a white gravy.. add cheese and allow cheese to melt.
taste cheese sauce and adjust salt and pepper.
put cauliflower in sprayed baking dish. pour sauce over the cauliflower ..
mix butter, bread crumbs, and paprika and sprinkle on top
bake for 30-35 minutes at 350°.