Set a box grater over/inside your slow cooker insert. Grate the sweet potato directly into the insert. Grate the yellow onion directly into the insert as well.
Rinse the red lentils thoroughly, until they stop foaming up in the strainer. Shake the excess moisture off and transfer them to your slow cooker insert.
To the insert, add the garlic, ginger, crushed tomatoes, coconut milk, water, turmeric, cayenne, mustard seeds, cumin, curry leaves, and black pepper. Give it all a good stir. In all honesty it’s going to look sort of gross at this point.
Place the insert into your slow cooker unit, lock the lid on top, and set the cooker to “high.” Let the soup cook for 4 hours.
Once the 4 hours are up, carefully remove the lid and stir the soup up a bit. Add the spinach, lime juice, and enough salt to season to your liking. Stir the soup one more time and lock the lid back on. Keep the soup going on the “high” setting until the spinach is wilted.
Serve the golden lentil soup hot with chopped cilantro.