Coat the bottom of a large pot with the oil and place it over medium heat. Give the oil a minute to heat up, then add the chopped leeks. Cook the leeks for 6 to 8 minutes, until softened and just starting to brown, stirring occasionally.
Stir in the garlic and cook it with the leeks for about 1 minute, until very fragrant.
Stir in the broth, cannellini beans, potatoes, thyme, and bay leaf. Raise the heat and bring the liquid to a boil. Lower the heat and let the soup simmer until the beans are very soft and the potatoes are fork tender.
Stir in the spinach and let the soup simmer for 1 to 2 minutes longer, until the spinach has wilted.
Remove the pot from heat. Remove the bay leaf, then stir in the lemon juice and zest. Season the soup with salt and pepper to taste.
Ladle into bowls, then sprinkle each bowl with fresh chives. Serve and enjoy.
