Step 1

Cook garlic, capers, and oil in a large saucepan over medium heat, stirring often, until garlic is golden, 4–6 minutes. Add onion, farro, and red pepper flakes and cook, stirring occasionally, until onion is slightly softened, about 3 minutes.

Step 2

Add 6 cups cold water to pot and bring to a boil. Reduce heat to medium-low and simmer, partially covered, until farro is tender, 20–25 minutes. Add celery and cook, uncovered, until almost tender, about 5 minutes. Stir escarole and Parmesan into soup and continue to cook until escarole is wilted, another 4 minutes. Remove from heat and season with salt.

Step 3

Do Ahead: Soup can be made 4 days ahead. Transfer to an airtight container and chill.