Par boiling the zucchini shell even for 5 minutes makes it so soft that you essentially have a flat piece of squash instead of a nice shell. There is nothing wrong with that but if you don't par boil you will have a nice "boat" in which to put the meat mix that will cook nicely in the oven. Otherwise a great simple recipe that makes use of those big zucchinis that I used to put in the compost pile.