Wash the zucchini. Cut half of it into very thin slices (with a mandolin, for example) and set aside to garnish. Dice the remaining zucchini.
Heat the olive oil over medium heat and fry the onion 5 minutes. Add garlic and diced zucchini and fry one minute until garlic is fragrant. Add curry powder, stir, then add water, stock cube, rice and salt to taste. Cover and simmer over low heat 30 minutes.
Meanwhile, place the zucchini slices in a bowl, season with salt and sprinkle with lemon juice. Let marinate until the soup is ready.
When the soup is cooked, puree with a blender. Reheat, add cayenne pepper and a little lemon juice. Serve, garnished with zucchini slices and a little pepper.