Cover the barley with cold water and let it soak for one hour; drain and set aside.
Heat the oil in a heavy bottomed saucepan over medium high heat. Add pancetta and cook until it starts to crisp up, about 3 or 4 minutes. Stir in the chopped onion, carrot and celery. Cook for another few minutes, or until the vegetables have softened slightly.
Add half of the beans and pour in enough water to cover. Stir in barley and remaining beans. Season with salt and pepper; simmer for 40 minutes, stirring regularly and adding more water if it's getting too thick for your taste.
Remove from the heat; allow to sit for 5 minutes before serving with a few rosemary leaves and a drizzle of olive oil.