Begin by draining and rinsing the soaked cannellini beans. In a large pot, cover the beans with water and bring to a boil for 3-4 minutes. Drain and set aside.

In the same pot, heat olive oil over medium-high heat. Add minced onion and chopped garlic, cooking until golden and fragrant, about 5-7 minutes.

Add the drained beans back to the pot with 6 cups of water or vegetable stock, bring to a boil, then reduce heat and simmer for about 2 hours.

After 2 hours, stir in sliced carrots and chopped celery, season with salt and pepper. Simmer for an additional 30-45 minutes.

Stir in the zest and juice of one lemon, cooking for 2 more minutes.

Ladle the soup into bowls, garnish with pepper and lemon zest if desired, and serve with crusty bread or salad.