Heat the olive oil in a large saucepan over moderate heat. Add the garlic and let it sizzle for about 1 minute, then add the sliced white and pale green parts of the leek and celery. Stir often while cooking until the vegetables soften, about 8 minutes. Do not let them color. Then pour the diluted stock into the saucepan and bring to a simmer.
Add the cream to the vegetable mixture and bring to a simmer. Add the peas and return to a simmer. Adjust the heat to maintain a gentle simmer and cook until the peas are just tender, about 8 minutes. Add the parsley and thyme and simmer for 1 minute longer, then remove from the heat. Transfer in batches to a blender with the ascorbic acid and puree, adding more of the stock-water mixture as needed to thin the soup to a pleasing consistency. Pour soup into a bowl set in an ice bath and stir until cooled. (we want to cool it down as quickly as possible) Serve soup with optional garnishes.
Chef's Tip: Ascorbic acid helps keep the green color in pureed herbs and vegetables. If you have vitamin C tablets on hand, just crush one with a rolling pin and add it to the mixture in the blender. Note that you'll need a blender to puree the soup. A food processor doesn't make it smooth enough.