In a large pot, sauté the onions with a few tablespoons of water for 7 to 10 minutes until the onions are translucent.
Now add the garlic, ginger, and curry powder. Stir and sauté for another minute.
Bruise the lemongrass stalk halves by crushing them with the butt of your knife or a pestle. Place them in the pot along with the kaffir lime leaves (if using), red lentils, and broth.
Bring to a boil. Once boiling, reduce to a simmer, cover, and simmer for 20 minutes.
Drain the cashews, discard the soaking liquid, and place in a blender. Remove the kaffir lime leaves if you used them and transfer the soup to the blender. Blend it all together until smooth. Do this in stages if needed. Return the soup to the pot and add the lime zest, lime juice, and tamari. Taste and add more lime or tamari as needed. Add more broth to thin the soup if desired. Serve and garnish with cilantro and scallions/green onion.