Heat the olive oil in a large pan. Add the onion and cook until tender, about 5 minutes.
Add the carrots, celery, garlic, thyme and sage and cook until the rest of the vegetables are slightly softened, about 5 more minutes.
Add in the sherry or white wine to deglaze the pan, along with the salt and freshly ground pepper. Let the wine reduce for a few minutes.
Add the potatoes and the vegetable broth. Cover and bring to a boil. Then, reduce the heat and let simmer until potatoes are tender, about 5-10 minutes. Be careful not to overcook them.
Add the vegetables to the base of a 9×13″ baking dish.
Preheat oven to 350 degrees F.
Make the crust. Place the flour, baking powder and salt in the base of a food processor fitted with a dough blade (Can also use a large bowl). Pulse together, then add in the mashed sweet potatoes and process until just combined. Add in the butter, one tablespoon at a time and pulse a few times to combine. Slowly drizzle in the water, 1 tablespoon at a time, until dough forms a ball. Remove from processor and dust with flour.
Roll out the dough to a thin layer, you may need to dust with additional flour to prevent sticking.
Cover casserole dish with dough and pinch edges all the way around until sealed.
Make several small slits in the dough. Bake until crust is golden-brown, about 35-40 minutes.
Alex Caspero is a Registered Dietitian, New York Times Bestselling Chef, and mom of two. She aims to cut through the nutrition noise by providing real-life, nourishing tips for body and mind. Learn more about Alex.