Add the oil and garlic to a medium pot over medium-low heat. Once the garlic starts to sizzle, cook it for 30 seconds; do not let it burn.
Add the beans and pepper to the pot and stir to coat. Cover the pot with a lid and cook for 5 minutes.
Add the broth, bring it to a simmer, and cook for 5 minutes more.
Add the spinach to the pot one handful at a time, stirring and allowing each addition to wilt before adding more. Once all the spinach is wilted, cook the soup for 1 minute more.
Remove the pot from the heat. Scoop out about half of the soup and set aside.
Using an immersion blender, purée the soup still in the pot until the spinach is broken down and the mixture is creamy and speckled with green.
Return the reserved soup to the pot, add the lemon juice (if using), and season with salt and pepper to taste.
Serve with bread, if desired.
