*Note: I used tamarind syrup instead of paste in this recipe. Two tablespoons tamarind syrup was stirred into 1/2 cup water. You don't need to let the mixture stand or drain it. If using tamarind paste, follow directions in Step #2:
Dissolve tamarind paste in 1/2 cup water. Let stand 30 minutes. Strain.
Cut potatoes into 1/2" slices.
Heat 2 tablespoons oil in heavy large skillet over medium heat. Add potatoes and cook until crisp and brown on both sides, stirring frequently, about 10 minutes. Remove from skillet and set aside.
Heat up another 1 tablespoon of oil over medium heat. Add ginger and garlic and cook 1 minute.
Stir in cayenne pepper or pepper flakes, turmeric, cardamom, fennel, cinnamon and black pepper. Cook until fragrant; about 1-2 minutes.
Add potatoes back to the skillet; stir gently to incorporate the spices and pour in the tamarind liquid. Arrange the crispy onions on top but do not stir into the mixture.
Cover, bring to boil, the reduce heat and simmer until sauce has thickened slightly, about 15 minutes. Season with salt.
If desired, garnish top of potatoes with fresh cilantro and a dollop of thick yogurt.