Dissolve tamarind paste in 1 cup water. Let stand 30 minutes. Strain. Boil potatoes until just tender. Cool completely, then peel. Using wooden skewer, pierce each potato in 4 places. Heat oil in heavy large skillet over medium heat. Add onion and cook until crisp and lightly browned, stirring frequently, about 10 minutes. Remove from skillet. Add potatoes and brown well on all sides. Remove from skillet. Pour off all but 1 tablespoon oil from skillet. Add ginger and garlic and stir 1 minute. Stir in pepper flakes and turmeric. Blend in 2 tablespoons water. Mix in onion. Add potatoes, tamarind liquid and remaining 14 tablespoons water. Stir in cardamom, fennel, cinnamon and pepper. Cover and simmer until sauce has thickened slightly, about 15 minutes. Season with salt.