Preheat oven to 400℉ (200°C). Line a baking sheet or sheet pan with parchment paper and spray with cooking spray. Roughly chop the cauliflower and onion. Remove the thyme leaves from the stems. Cut the top end off of a whole bulb of garlic.
Add the cauliflower, onion, and garlic to the sheet pan in a single layer and coat them with olive oil, paprika, cumin, coriander, salt, and pepper. Sprinkle the thyme leaves on top. Bake in the preheated oven for 30-35 minutes. Oven times will vary, cauliflower is done cooking when it turns a golden brown color.
When the vegetables are done roasting, remove them from the oven and add everything to a blender. *To add the garlic cloves, carefully squeeze them out of the skin being careful not to get the skins in the blender.
Drain and rinse the white beans and add them to the blender along with the broth and lemon juice. Blend everything until smooth using a spatula to push down the sides as needed.
To heat the soup, pour it into a large soup pot or dutch oven and heat on medium heat until it starts to bubble. Then lower to medium-low or low heat for another minute or two. If it's too thick, add 1 more cup of broth. Add salt to taste.
Hint: if you don't have a high-speed blender, you can add the soup ingredients including the roasted vegetables, beans, broth, and lemon juice to a large pot and use a hand-held immersion blender to blend it.