Step 1Wash the lentils: Place the lentils in a pot or bowl and cover with water. Stir or swish the water with your hand (the water will turn murky), drain and repeat the process until the water is clear. Drain and place in a large pot.
Step 2Chop the sweet potato, carrot and onion into coarse pieces and add them to the pot, along with the garlic. Add the 2 quarts of water. Bring to a boil over high heat, then reduce the heat to a gentle simmer, partially cover and cook until the vegetables and lentils are softened, 20 to 25 minutes.
Step 3Stir in the coriander, turmeric and cumin, then stir in the salt. Cook an additional 5 minutes to marry the flavors. Remove from heat and puree the soup (either using a hand blender or in batches using a standard blender).
Step 4Divide the soup into serving bowls. Garnish each serving with a drizzle of coconut milk or half-and-half to create a design on the surface of the soup. Garnish with chopped chives or cilantro if desired. Serve immediately.