6 oz pancetta, one 1-inch slice cut into 1-inch cubes
1 lb dried cannellini beans, rinsed and picked over
1 large onion, unpeeled and halved pole to pole
4 medium cloves garlic, unpeeled
1 bay leaf
1 t table salt
¼ cup extra virgin olive oil, plus extra for serving
1 small onion, diced medium
1 medium garlic cloves, minced
1 sprig of fresh rosemary
Salt and freshly ground black pepper
Balsamic Vinegar, for serving
In large, heavy-bottomed Dutch oven, cook pancetta over medium heat until just golden, 8 to 10 minutes. Add 12 cups water, beans, halved onion, unpeeled garlic, bay leaf, and 1 teaspoon salt; bring to boil over medium-high heat. Cover pot partially; reduce heat to low, and simmer, stirring occasionally, until beans are almost tender, 1 to 1 1/4 hours. Remove beans from heat, cover, and let stand until beans are tender, about 30 minutes. Drain beans, reserving cooking liquid (you should have about 5 cups; if not, add more water); discard pancetta, onion, garlic, and bay leaf. Spread beans in even layer on baking sheet and cool.
While beans are cooling, heat oil in now-empty Dutch oven over medium heat until shimmering; add diced onion and cook, stirring occasionally, until softened, 5 to 6 minutes. Stir in minced garlic and cook until fragrant, about 30 seconds. Add cooled beans and cooking liquid; increase heat to medium-high and bring to simmer. Submerge rosemary sprig in liquid, cover and let stand off heat 15 to 20 minutes. Discard rosemary sprig and season to taste with salt and pepper. Ladle soup into individual bowls, drizzle each bowl with extra-virgin olive oil, and serve, passing balsamic vinegar separately.