Rinse leeks well and pat dry. Halve lengthwise and chop. In a large saucepan, melt butter over medium high. Add 1 cup chopped leeks and cook, stirring, until softened, 3 minutes.
Add broth, 1/2 cup water, and thyme and bring to a boil. Reduce to a simmer and whisk in mashed potatoes. Simmer until warmed through; season with coarse salt and ground pepper.
I love using leftovers and was able to put this together in ten minutes! I used "Better than Bouillon" instead of chicken broth, a pinch of dried thyme, Yukon golds, and it tastes like a hot vichyssoise because I had made my mashed potatoes with lots of cream the night before. I will make this soup again... tasty!
The amount of liquid is way too much, the thyme is overwhelming, and the leeks don't get soft enough to make a pleasant soup. Stringy, runny, unpleasant.