Take your second cauliflower head and cut it down into mini florets, of similar size to the pasta shape you are using, .5-1cm in diameter. Add macaroni pasta to the boiling pasta water. (Elbow macaroni takes 7 minutes to get to al dente, and given it will cook more in the oven, aim for a bit under 7 minutes.) 3 minutes into the boil, add the mini florets, and finish boiling.