Heat the olive oil in a large pot over medium heat. Add the onion and cook 5 minutes until soft. Add the garlic and cook 1 more minute.
Add the broccoli, white beans, broth, and cashews. Bring to a boil, then reduce heat and simmer 12–15 minutes, until the broccoli is very tender.
Blend until smooth, using an immersion blender right in the pot, or in batches in a regular blender. (Leave it just a little chunky if you'd like some small broccoli pieces to remain.)
Stir in the lemon juice, then season with salt and pepper to taste.