01 - Set the oven to 400°F to prepare for roasting the vegetables.
02 - Arrange fennel slices, onion, and garlic cloves on a large baking sheet. Drizzle with 1 tablespoon olive oil and season lightly with salt and pepper. Toss until the vegetables are evenly coated.
03 - Roast in the oven for 25 to 30 minutes, turning once halfway through, until fennel is caramelized and tender.
04 - Heat 1 tablespoon olive oil in a large soup pot over medium heat. Add carrot and celery and sauté for 5 minutes until softened.
05 - Transfer roasted fennel, onion, and garlic to the pot. Add drained cannellini beans, thyme, and vegetable broth. Bring to a boil, then reduce heat and let simmer for 15 minutes to develop flavor.
06 - Remove the pot from the heat. Use an immersion blender to blend until smooth and creamy, or carefully purée in batches with a blender.
07 - Stir in heavy cream, lemon zest, and lemon juice. Taste and adjust seasoning with salt, pepper, or more lemon as needed.
08 - Gently reheat if necessary, then ladle into bowls. Garnish with fennel fronds, a drizzle of olive oil, and serve with crusty bread if desired.