Preheat the oven to 425 degrees F. Lightly grease a sheet pan with oil.
In a large bowl, whisk together the tahini, olive oil, cumin, red pepper flakes, salt and pepper. Toss the cauliflower in the mixture.
Arrange the cauliflower in a single layer on a baking sheet. Roast for 20-25 minutes, flipping halfway through cooking, until browned.
For the soup, heat 1 tablespoon olive oil in a large saucepan over medium heat. Then sauté the onions until they start to soften, about 4-5 minutes.
Add the garlic, salt and pepper and cook for 30 seconds to 1 minute until fragrant.
Add the roasted cauliflower (saving 6-8 cauliflower florets for garnish), white beans, vegetable broth and parsley. Bring the mixture to a boil. Reduce the heat and simmer for 10 minutes.
Carefully transfer the soup to a blender and puree until smooth.
Return the soup to the saucepan and warm low heat. Stir in the lemon juice.
Divide the soup into bowls. Top with the reserved roasted cauliflower and chopped parsley.