Step 1

Tie oregano, rosemary, and bay leaves together with kitchen twine.

Step 2

Heat oil in a large heavy pot over medium. Add pancetta, if using, and cook, stirring often, until browned around the edges, about 5 minutes. Add onion, leek, carrots, celery, garlic, and red pepper flakes; season with salt. Cook, stirring often, until onion is translucent and carrots are tender, 10–12 minutes. Add tomato paste and cook, stirring to coat, until slightly darkened, about 3 minutes.

Step 3

Add tomatoes, crushing with your hands as you go, then herb bundle, Parmesan rind, if using, and 6 cups water or reserved bean cooking liquid, or a combination. Season with salt and pepper and bring to a boil. Add potatoes, reduce heat, and simmer until potatoes are tender and flavors have melded, 20–25 minutes. Add kale and beans; cook until kale is tender, about 5 minutes. Discard Parmesan rind and herb bundle.

Step 4

Do Ahead: Soup can be made 2 days ahead (or 2 months if frozen). Let cool; transfer to airtight containers and chill.

Step 5

Pulse garlic and basil in a food processor until finely chopped. Transfer to a small bowl and stir in oil, Parmesan, and lemon zest; season with salt. Serve soup topped with pistou.