Best Vegetarian Kale Soup

In a large pot over medium heat, heat oil. Add onion, celery, and leek and cook until slightly soft, 6 minutes. Add garlic, thyme, and red pepper flakes (if using) and cook until fragrant, 1 minute more. Season with salt and pepper. Add broth, water, and beans and bring to a simmer. Stir occasionally, mashing some beans in the pot to thicken soup. Let simmer 15 minutes, then stir in lemon juice and kale. Cook until wilted, 3 minutes. Garnish with Parmesan before serving.