You have plenty of time while the onion and zucchini are browning and while broth is reducing to chop up all the root vegetables for the casserole. To make this ahead you can either do steps 1-6 the day before, and then bake before serving or fully cook the day before (bake for 40-45 minutes, until the root vegetables are cooked al dente) and then reheat for 30 minutes in the oven before serving.