Melt the butter in a large heavy-bottomed pot over medium-high heat. Add the onion, celery, and leek and season with salt and pepper. Saute the vegetables until they are softened but not browned, about 5 minutes. Add the curry powder, stir, and cook for 2 minutes more.
Add the stock and herb sachet to the pot. Bring to a boil over high heat, then lower the heat and let simmer until the vegetables have completely softened, 10 to 12 minutes. Add the peas and simmer for an additional 10 minutes.
Use tongs to remove and discard the herb sachet.
Use an immersion blender to puree the soup in the pot, or puree it in batches in a standing blender, then return the soup to the pot over medium heat. Stir in the creme fraiche, being careful not to let the soup return to a boil. (If you like, reserve a few tablespoons of the creme fraiche to drizzle over each serving.)
Divide soup among individual bowls and serve at once, topping each serving with a drizzle of creme fraiche, if desired, and a few pieces of frizzled ginger.
Pour the oil into a heavy-bottomed stockpot or lobster pot. Add all the other ingredients and cook, stirring, over medium-low heat until the vegetables have all softened but not colored, about 15 minutes. Pour in 6 quarts water, raise the heat to high, and bring the water to a boil, then lower the heat and let the liquid simmer for 2 hours.
Carefully strain the stock through a fine-meshed strainer set over a large bowl or pot, pressing down on the solids with a wooden spoon to extract as much liquid and flavor as possible. Let the stock cool, then refrigerate for up to 1 week or freeze in batches to be thawed and used as needed.
Pour oil into a small pot and heat to 265 degrees F. Shave the ginger into a fine julienne using a Japanese mandoline, vegetable slicer, or very sharp thin-bladed knife. Adds the ginger crisps to the oil, frying them until crispy, about 1 minute; use a slotted spoon to remove them to a paper towel-lined plate to drain. Salt as soon as it hits the paper. These crisps can be held at room temperature for up to 2 hours.
Recipe courtesy Michael Lomonaco, "Nightly Specials", William Morrow, 2004