4 tablespoons extra-virgin olive oil
1 head garlic, halved crosswise
10 ounces Parmesan rinds, enclosed in cheesecloth or muslin
1 lemon, rind peeled into thick strips with a vegetable peeler, then halved and juiced
8 sprigs fresh thyme
½ teaspoon red-pepper flakes
1 pound hearty greens (such as escarole, kale, mustard greens or rapini), stemmed and torn into bite-size pieces
Kosher salt and black pepper
2 (15-ounce) cans white beans (such as Great Northern or cannellini) or chickpeas, drained and rinsed
Freshly grated Parmesan, for serving
- To make the Parmesan broth: In a large pot, heat 2 tablespoons olive oil over medium-high. Add the garlic, cut-sides down, and cook until golden brown, 2 to 3 minutes. Add the Parmesan rinds, lemon rinds, thyme and 8 cups water and bring to a boil. Reduce heat to a simmer and cook, uncovered, until the broth tastes full and rich and is reduced by half, 1 to 1 1/2 hours. (You will have about 4 cups of broth.) Reserve half the garlic and half the lemon rinds, then strain the broth. (Once cool, the broth will keep for 1 week refrigerated or 3 months in the freezer. If freezing, leave a bit of space between the broth and the lid of the container, as the broth will expand.)
- To make the soup: Squeeze the garlic cloves to release them from the reserved head. Finely chop the reserved lemon rind. In a large pot, heat the remaining 2 tablespoons olive oil over medium-low. Add the garlic cloves and the red-pepper flakes and cook, breaking up the garlic with your spoon, just until fragrant, 2 to 3 minutes.
- Increase the heat to medium, add the greens and the chopped lemon rind and a pinch each of salt and pepper. Cook, stirring, until the greens are wilted, 1 to 2 minutes.
- Add the beans and all of the Parmesan broth and bring to a boil. Reduce the heat to a simmer, cover, and cook until the greens are silky and the beans and broth are warmed through, about 10 minutes.
- Remove from heat and stir in the lemon juice to taste. Season to taste with salt and pepper. Serve with black pepper and grated Parmesan on top.