Pour the lentils into a fine-mesh sieve and rinse under cool running water until the water draining is no longer cloudy. Dump the lentils into a slow cooker.
Finely chop the shallots; mince or grate the garlic. And them both to the lentils. Using a microplane, grate the ginger into a fragrant heap of pulp and juice. Reserve 1 teaspoon of this to stir into the yogurt topping; add the rest to the slow cooker.
Sprinkle in the cumin and the curry powder; dump in the sweet potato puree and give everything a stir.
Shake the coconut milk well, then open the can and pour it over the soup ingredients. Finish by adding the vegetable stock and give the pot a final stir. Cover with a lid and set to High for 5 hours. Easy, right?
Just before serving, stir in the salt and the lime juice. Taste and adjust seasoning to taste; it may be necessary to add more salt.
Stir together the yogurt and the reserved grated ginger. Serve alongside the lentil soup, adding a dollop to the top of the bowls. Garnish with chopped cilantro, toasted coconut flakes or a drizzle of olive oil, if desired
Note that slow cookers may vary, so if you find that yours tends to cook meals faster than usual, begin checking to the soup around 4 hours. You are looking for a tender lentil in a fragrant broth.