Bring the lentils and bay leaf to a boil in 3 cups water, then reduce the heat and simmer until the water cooks away, 15 to 18 minutes. Skim any by-product away as the lentils cook.
Meanwhile, cover the potatoes with water in a small pot and bring to boil. Salt the water and boil for 5 minutes. Drain the potatoes and run under cold water to cool. Drain well and transfer to a large bowl.
Drain the lentils and run under cold water to cool. Drain well and add to the potatoes.
Meanwhile, heat the vegetable oil in a skillet and saute the onions, garlic, ginger and chile pepper until softened. Stir in the tamarind paste and spices to toast them, then remove from the heat.
Add the spice mixture to the lentils and potatoes. Gently stir in the cilantro, spinach and lime juice. If the salad seems dry, add a little more oil. Serve.
You can use 2 tablespoons store-bought curry paste (such as Patak's) in place of the tamarind, turmeric, cumin, coriander, powdered mustard and cinnamon.