Roast the garlic: Heat the oven to 400°F (200°C). Slice the top off the garlic head to expose the cloves. Drizzle with olive oil, wrap tightly in foil, and roast for 35–40 minutes until soft and golden.
Let cool slightly.
Sauté the aromatics: In a large pot, warm 2 tablespoons olive oil over medium heat. Add onion, celery, and carrot (if using). Season with a pinch of salt and cook 6–8 minutes, stirring, until tender and translucent.
Add herbs and seasonings: Stir in thyme, rosemary, black pepper, and the bay leaf.
Cook 1 minute until fragrant.
Add beans and broth: Add the rinsed beans and pour in the broth. Squeeze the roasted garlic cloves out of their skins directly into the pot. Stir to combine.
Simmer: Bring to a gentle boil, then reduce to a simmer.
Cook 10–15 minutes to let the flavors meld. Taste and add salt if needed.
Blend to your preferred texture: Remove the bay leaf. Use an immersion blender to blend until smooth and creamy.
For some texture, blend only half the soup. If using a stand blender, work in batches and vent the lid.
Finish and brighten: Stir in the lemon juice and a pinch of red pepper flakes if using. Adjust seasoning with more salt, pepper, or lemon to taste.
If too thick, add a splash of broth or water until it’s where you want it.
Serve: Ladle into bowls and garnish with chopped parsley or chives. Drizzle with a little olive oil and serve with crusty bread.