Heat the olive oil over medium heat in a large, heavy soup pot and add the onion. Cook, stirring, until it is tender, about 5 minutes. Add a generous pinch of salt, the garlic and the zucchini and stir for about a minute, until the garlic smells fragrant. Add the curry powder, stir together, and add the stock or water, the rice and salt to taste. Bring to a boil, reduce the heat, cover and simmer 30 minutes. Taste and adjust salt.

Purée the soup with an immersion blender or in batches in a blender, taking care to remove the lid or take out the center insert and to cover with a towel to avoid hot splashes. Return to the pot, heat through, add pepper and cayenne to taste and stir in the lemon juice.