Place the tamarind in a bowl. Add 2 cups hot water. Mash with a fork. Soak then drain with a sieve to remove the seeds. It should make 1 cup of strained tamarind.
Wash the lentils very well. Soak in water 15-20 minutes. Drain.
In the meantime, roughly chop the onion. Peel the potato and chop into 3/4-inch pieces. Soak the potatoes in water. Mince the garlic.
Heat 3-4 tablespoons light oil in a skillet. Add the onions and saute until golden. Add the turmeric and garlic. Stir, then add the tomato paste, curry powder, several grinds of black pepper, and the cayenne or Aleppo pepper.
Add the lentils and several pinches of salt. Saute the lentils 1-2 minutes until they start to smell toasty.
Transfer to a large broad Dutch oven. Drain the potatoes and add them to the pot.
Boil water and add 4 cups to the lentils. You may add more pinches of salt as needed.
Simmer on medium-low about 15 minutes. When the lentils and potatoes are soft, add the strained tamarind. Stir occasionally. Taste for salt and red pepper.
Serve with basmati rice, preferably with tahdig.
