An irresistible vegetarian main course can take a little extra love and time, but food editor Melissa Roberts would be the first person to tell you it's worth it. Fortunately, all the components of this shepherd's pie—from the heady vegetable stew with its red wine gravy to the crown of fluffy mashed potatoes and celery root—can be made ahead. Protein-rich seitan (pronounced "SAY-tan," it's sometimes labeled "wheat gluten") has long been used as a meat substitute by Asian cooks, who know how to coax the most out of its satisfyingly chewy texture. Seitan doesn't have a strong taste of its own, but it soaks up the flavors of whatever it's cooked with.
• You can use more potatoes in place of the celery root.
• Stew and potato topping can be made 1 day ahead (reserve an additional cup potato-cooking water) and chilled separately. Reheat potato topping slowly in a microwave or in a heatproof bowl set over a pot of simmering water, adding enough cooking water to reach desired consistency. Bring stew to a simmer over low heat before topping with warm potato mixture.
