The farroan ancient whole grain rich in iron and fiber—and butternut squash make this slow-cooker soup hearty and filling. If you're looking for an easy way to work whole grains into your diet, this is it.
Heat the oil in a large nonstick skillet over medium-high. Add the mushrooms and ¼ teaspoon of the pepper; cook, stirring often, until the mushrooms are browned, 8 to 10 minutes. Add the onions and celery; cook, stirring often, until the onions and celery are slightly softened, about 4 minutes.
Transfer the mushroom mixture to a 5- to 6-quart slow cooker. Stir in the squash, water, stock, and farro. Cover and cook on LOW until the farro and vegetables are tender, about 4 hours.
Just before serving, add the spinach, parsley, basil, vinegar, salt, and remaining ¼ teaspoon pepper, stirring until the spinach wilts. Ladle the soup into bowls; top evenly with the Parmigiano-Reggiano.
Multicooker Directions: Complete Step 1. In Step 2, transfer the mushroom mixture to the inner pot of a 6-quart multicooker. Stir in the squash, water, stock, and farro. Lock the lid; turn Pressure Valve to "Venting." Cook on SLOW COOK [Normal] until the farro and vegetables are tender, 4 hours. Complete Step 3.