Roast cauliflower - Preheat the oven to 425 degrees F. On a large baking sheet, toss the cauliflower with 2 tablespoons of olive oil. Arrange in a single layer on the pan and sprinkle with salt. Bake for 25-35 minutes, flipping halfway through, until caramelized and golden.
When the cauliflower has about 10 minutes of bake time left, heat the other tablespoon of olive oil in a large pot over medium heat. Add the onion and sauté for 3-4 minutes, until translucent, then add the garlic and cook for 1 more minute.
Add the cannellini beans, thyme, rosemary, broth and the roasted cauliflower, saving a few of the prettiest pieces for garnish, if desired. Bring to a boil then reduce heat and simmer for 10 minutes.
Transfer the soup in batches to a blender (do NOT overfill your blender!) and blend until smooth and creamy. For more texture and less of a smooth soup, use an immersion blender to just blend a little bit. Return the the pot.
Stir in the lemon juice and add salt and pepper, to taste.
Serve in bowls with a few croutons or florets on top, with a side of crusty bread for dipping. Enjoy!