Tuscan black kale, or cavolo nero, has impressive stores of calcium and vitamins A and C. A sprinkle of freshly grated Parmesan gives the soup creaminess and tang without excessive calories.
Bring 8 cups water to a boil in a large pot. Reduce heat, add barley, and simmer until just tender, about 1 hour.
Add 2 cups stock to pot, and bring to a boil. Reduce heat, and add onion, garlic, red-pepper flakes, and salt. Season with pepper. Simmer until barley is soft, about 20 minutes.
Add beans with liquid, kale, and remaining 2 cups water and 1 cup stock. Simmer until kale is tender and cooked through, about 7 minutes.
Divide among 4 bowls. Sprinkle with Parmesan and lemon zest, and season with pepper.