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Puree this mixture with the ginger and green chilli in a blender until smooth. Return to pan and bring to simmer. Add salt, sugar and tamarind and stir to dissolve the tamarind. Remove from heat.
Meanwhile, heat oil in a 6 inch / 15 cm skillet over a medium low heat. Fry black mustard seeds for a few seconds. As soon as the seeds start popping, remove from heat and pour contents over lentil mixture. Simmer lentils 2-3 more minutes. Stir in 5 spice. Cover and let stand for a few minutes to develop the flavours.
Garnish with lemon wedges and fresh cilantro.