Peel the onion and chop it coarsely. Heat the olive oil in a large saucepan. Add the onion and sage, and sauté them over medium heat for 5 minutes, or until the onion is translucent. Meanwhile, halve the bell peppers lengthwise. Clean one half of a bell pepper, removing the seeds and white pith, and dice it into 1½ cm pieces. Set it aside. Clean the remaining bell peppers, chop them coarsely, and add them to the onions in the saucepan. Add ¾ tsp salt, bay leaves, ground cumin, sugar, and chili. Sauté for an additional 5 minutes. Pour in the broth and bring it just to a boil. Cover the saucepan and let it simmer over very low heat for 15 minutes. Remove the bay leaves from the soup once the bell peppers are tender. Blend the still-warm soup with an immersion blender until completely smooth. This may take a few minutes. Allow the soup to cool slightly. Stir in the celery dice, red bell pepper dice, lemon zest, and garlic into the warm soup. Let it cool to room temperature, then refrigerate for a couple of hours or overnight. Take the soup out of the fridge 30 minutes before serving. Stir thoroughly, and season with salt and pepper to taste. Pour the soup into individual bowls, generously sprinkle with chopped basil and parsley, add a dollop of sour cream per person, and finish with a drizzle of olive oil.