Sweet Potato-Apple Soup

2 T butter
1 onion, diced finely
3 stalks celery, sliced
2 lbs sweet potatoes (about 5), peeled and coarsely grated
2 large apples, peeled, cored, and coarsely grated
5 C vegetable stock
1-1/2 t grated lemon or orange zest, divided
1/2 t salt, or to taste

You can make this in a pressure-cooker, or in a Dutch oven.

Saute the onions in the butter for a couple of minutes. Add the remaining ingredients, saving 1/2 t of the orange zest, and stir to combine.

Pressure cook for 4 minutes, then allow the pressure to drop naturally. When you open the pressure cooker, you can puree part of the soup or leave it as is. Stir in the rest of the orange zest and salt to taste before serving.