Save the porcini soaking water - it becomes the stock for the gravy. Pour it in carefully and leave any grit behind at the bottom of the bowl. Salt to taste, and go easy if your vegetable broth is already salted. Store leftovers in an airtight container in the fridge for up to 4 days or freeze for up to 3 months; reheat gently and loosen with a splash of broth if the gravy has thickened too much.