Heat olive oil in a large pot or Dutch oven over medium heat.
Add chopped onion, carrots, and celery. Cook until softened, about 5-7 minutes.
Stir in minced garlic, dried thyme, and red pepper flakes (if using). Cook for another minute until fragrant.
Pour in the vegetable broth and add the rinsed and drained cannellini beans.
Bring the soup to a boil, then reduce heat and simmer for 15 minutes to allow flavors to meld.
Add the chopped kale to the pot. Stir until wilted, which should take about 3-5 minutes.
Season the soup with salt and black pepper to taste.
Serve hot.
Notes
This soup is even better the next day as the flavors deepen. Feel free to add other vegetables like potatoes or zucchini. For a creamier texture, you can blend a portion of the soup before adding the kale.