Heat the olive oil in a large pot over medium heat. Add the onion, carrots and celery and cook for 7–9 minutes, stirring occasionally, until softened and lightly golden.
Add the garlic, rosemary, thyme and red pepper flakes. Cook for 1 minute, stirring frequently, until fragrant.
Add the white beans, potato and vegetable broth. Season with the salt and black pepper. Bring to a boil, then reduce the heat to maintain a gentle simmer. Partially cover and cook for 15–18 minutes, or until the potato is very tender.
Remove the rosemary and thyme sprigs. Transfer about 4 cups of the soup, including plenty of beans and potato, to a blender. Blend until smooth. Alternatively, use an immersion blender to partially blend the soup directly in the pot, leaving some beans and vegetables whole.
Return the blended soup to the pot and stir to combine.
Stir in the kale and lemon juice and remove from the heat. Let sit a few minutes until the kale is tender. Taste and adjust the salt, pepper and lemon juice as needed.
Serve hot with fresh parsley and a drizzle of olive oil, if desired.