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.
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.