Photograph: Jan Baldwin
Heat the olive oil in a large saucepan set over a low-medium heat. Add the onion, carrot and celery to the pan and cook gently, stirring often, for about 10 minutes, until softened. Turn up the heat, stir in the tomato purée, garlic and chilli flakes and cook for a minute or so. Add the cannellini beans to the pan with the stock and thyme leaves and bring to the boil. Reduce the heat slightly and simmer for 5 minutes.
Remove the saucepan from the heat and tip half of the soup into a bowl. Using a handheld blender whiz the remaining soup to a purée. Return the rest to the pan and stir in the kale. Season and bring back to a simmer for 15 minutes, stirring often until the kale is completely tender. Stir in the lemon juice to lift the flavours.
To make the parmesan toasts, preheat the grill to medium. Brush the baguette slices with the olive oil and space out on a baking sheet. Grill for a couple of minutes, until pale golden. Turn the toasts over and sprinkle the non-grilled sides with half the grated parmesan. Return to the grill until the cheese is melted and bubbling.
Serve the hot soup scattered with the reserved parmesan, with the parmesan toasts.
Get ahead: make the soup the day before, chill. Reheat before serving and add the lemon juice to serve.