Cut off the dark green part of the leeks and then chop into smaller pieces (about 2".) Bring the water, the stock, and the dark green leek leaves to a boil, and then reduce heat to a simmer and allow leaves to steep for 30-45 minutes uncovered.
While the stock is simmering, peel and chop your potatoes and parsnips. Then slice up your leeks: holding the root end in your hand, carefully insert the knife into the leek just below the root and slice downward. Turn a quarter turn and do this again. Rinse well, pulling apart the layers, and then finely chop. Great way to remove the dirt and sand that is easily stuck between all the layers.
Toast your bread till crunchy but not brown (better to do in the oven, about 250 for 15 minutes, than the toaster) and break into big pieces.
Strain the solids from the simmering leek stock, pressing out any liquid, and then discard them. You'll need about 3-4 cups of stock, depending on how thick you'd like your soup.
In the pot where you made the leek stock, melt your oil or butter over medium heat. Add in the chopped white and light green leeks, and cook 5 minutes until slightly softened. Add in parsnips and potatoes for an additional 10 minutes. Add in the salt here, and adjust the heat to make sure the veggies don't brown.
Add in the stock and mashed potatoes if using, and cook until the potatoes and leeks can be pierced with a knife (about 20 minutes). Add in the bread and allow to soak in for 5 more minutes.
Blend in batches in your blender, or better yet use an immersion blender so you don't have to transfer to and fro. Taste, add pepper and adjust the seasoning. Serve with sour cream and some finely minced green onions or scallions on top.