Chop: Finely chop the onions, garlic, carrots, celery, and pepper.
Cook vegetables: Heat the oil in a large pot. Cook the vegetables with a pinch of salt, on medium heat, for about 7 minutes until softer. Add a splash of water and cook for further 3 minutes. The onions should be soft, as they will not soften anymore once you add the tomato paste. Add cumin and red pepper flakes and stir for about 30 seconds. Add tomato paste and stir well for about 1 minute.
Cook soup: Pour in about 2 cups vegetable stock. Drain the beans. Add them to the pot. Increase the heat, bring to a boil, lower the heat again and cook for 20 minutes.
Blend: Scoop about 4 cups and blend in a blender until smooth (Note 2). Return to the pot. Alternatively, use an immersion blender or potato masher to mash the soup partially; it should not be smooth but creamy yet chunky.
Thicken: Mix the cornstarch with cold water. Slowly add to the soup and let bubble for 1-2 minutes until thicker. For a thinner consistency, add more vegetable stock, and don't thicken the soup with cornstarch slurry. In this case, you might want to add more cumin and red pepper flakes as well. For a smoother consistency, you can blend the soup more thoroughly.
Adjust: Remove from the heat and adjust the taste with lemon juice and salt.
Serve: Serve hot, garnish with any of the suggested items.