Make the potatoes: Steam the sweet potatoes for about 40 minutes or until soft.
Sauté the ingredients: In the meantime, heat the olive oil in a pressure cooker. Sauté the onion and garlic until softened. Stir in the walnuts and toast until starting to brown. Stir in the spice mix, lentils and tomato paste and sauté until fragrant.
Cook the lentils: Pour the water into the pressure cooker and stir well. Season with salt and cook on high pressure for 8 minutes for a stovetop pressure cooker, 10 minutes for an electric pressure cooker (such as the Instant Pot).
Finish the filling: Quick-release the pressure. Stir in the tomato sauce and harissa if using (start with 1/2 teaspoon, then adjust to how spicy you like it) and simmer gently without the lid until the lentils are completely done and the sauce has thickened, about 5 minutes.
Stuff the potatoes: Pierce the potatoes with a pointy knife to check for doneness. When they're done, remove them from the steamer and set them on plates. Make space for the filling (I like to cut out a slice of the potato lengthwise), then spoon the lentil filling inside. It's OK if a little spills over - the more, the better! Top with cilantro and pomegranate to serve.
If you don't have a pressure cooker, you can cook the lentils on the stove in a Dutch oven: Start sautéing the ingredients before you steam the potatoes. As soon as the walnuts are getting toasty, get those potatoes into the steamer. Follow the recipe for the lentils as written, but simmer them for 25 minutes with the lid on before adding the tomato sauce.
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