Heat oil in a medium-large soup pot over medium heat. Add onions, carrots, and celery, then stir. Sauté until the vegetables are lightly softened and translucent, about 5 minutes.
Add the garlic, chili flakes, and rosemary to the pot. Stir and cook until fragrant, about 30 seconds. Stir in the navy beans. Pour in the vegetable stock and stir again. Bring the soup to a boil.
Once boiling, transfer half the soup to an upright blender. Add the lemon juice. Carefully increase the blender speed to high and blend until completely smooth. Return the blended soup to the pot. Season with salt and pepper.
Add the kale to the pot and bring the soup to a boil. Once the kale is slightly wilted and bright green, season again with salt and pepper if needed. Stir in the chopped parsley. Serve hot.
