In a large pot on medium-high, combine the curry paste, coconut milk and 6 cups water. Season with salt and pepper. Bring mixture to a boil. Meanwhile, peel and cut the sweet potatoes into 1-inch pieces.
Add the lentils and sweet potatoes to pot and return to a boil. Season with salt and pepper. Reduce to a simmer, cover, and cook 15 to 18 minutes, until potatoes are tender. In batches, stir the spinach into pot and cook 1 minute, until wilted. Serve immediately.
Tip: Garnish with chopped cilantro.