Heat the olive oil in a large pot over medium heat. Add the onion, carrot, and celery and cook for 7–8 minutes, stirring occasionally, until softened and beginning to color.
Add the garlic and cook for about 30 seconds, until fragrant. Stir in the tomato paste and Italian seasoning and cook for 1 minute, until the tomato paste darkens slightly.
Add the cannellini beans, vegetable broth, bay leaf, salt, and black pepper. Stir and bring to a gentle boil.
Reduce the heat and simmer uncovered for about 15 minutes, until the vegetables are tender and the flavors in the broth have come together.
Remove and discard the bay leaf. Carefully transfer about 2 cups of soup to a blender and blend until smooth. Return it to the pot and stir, leaving plenty of whole beans and vegetables for texture. Alternatively, use an immersion blender for a few short pulses.
Stir in the kale and simmer for 3–5 minutes, until tender but still green.
Remove the pot from the heat and stir in the lemon zest and lemon juice. Taste and adjust the salt and black pepper as needed. Add a little more vegetable broth if you'd like a thinner consistency.
