1 lb (450g) leeks (white and light green parts only)
1 TBS olive oil
2 cloves garlic, minced
2 tsp minced ginger
1 TBS freshly grated lemongrass or lemongrass paste
1 Thai chili, thinly sliced (optional)
2 lbs (900g) russet or yukon potatoes, peeled and chopped into 1/2-inch chunks
4 1/2 cups (950ml) vegetable broth
1/2 cup (120ml) water
salt and pepper, to taste
Chop off the ends of the leeks. Slice leeks in half lengthwise, and fan them out a little. Run leeks under water and clean out any dirt. Slice leeks into 1/4-inch thick half moons.
Heat olive oil in a heavy-bottomed pot over low heat. Add leeks and cook until their softened, about 8 minutes. Stir often to keep leeks from burning. Add garlic, ginger, lemongrass, and Thai chili and mix well. Add potato chunks, broth, and water.
Bring up heat to medium-high and bring broth to boil. Reduce heat and let soup simmer for 8 to 10 minutes, partially covered, until the potatoes can be easily pierced with a fork. Turn off heat and season soup with salt and pepper. Using an immersion blender or a high-speed blender, blitz soup until smooth.
Serve hot with cilantro, pistachios, and microgreens, if you like.