Heat the olive oil in a wide based pot or Dutch oven over medium heat. Add the onion, carrot, celery, garlic and chili flakes and cook 7-8 minutes, stirring occasionally, until partially softened. Season the vegetables with salt and pepper. Add the tomato paste and cook another 1-2 minutes until fragrant. Stir in the kale and cook until it starts to wilt, 3-4 minutes. Add the diced tomatoes, stock (or water), thyme, bay leaf and parmesan rind and raise the heat to bring to a simmer.

Meanwhile, pour about ¼ of the cannellini beans into a small bowl with a couple of tablespoons of the cooking liquid and mash them together with a fork to form a paste. Pour the paste along with the remaining whole beans into the soup and stir to combine. The mashed beans will help to thicken the soup as it cooks. Simmer the soup with the lid slightly ajar, about 25 minutes until the vegetables are softened but still al dente. Add the bread and simmer another 5-7 minutes, partially covered. The bread will start to dissolve into the soup and thicken it further.

Before serving, remove the thyme sprigs, bay leaf and parmesan rind. Adjust seasoning with salt and pepper. Spoon the ribollita into bowls and top with parmesan cheese and a drizzle of olive oil, if desired.