Tangy Arhar Daal with Tomato and Black Mustard

I’ve been obsessed with Arhar (Toor) daal of late. Its characteristically nutty flavour stands up to a lot of strong spices like black mustard and hing (asafoetida), and is lovely when balanced with a slight sweet-and-sour note. I experimented with sugar, tamarind, aamchur, and kokum in various permutations, and found that a combination of tomatoes and brown sugar, when used sparingly, does just the trick.

Ingredients 

2 cups Arhar (Toor) daal
2½ quarts (10 cups) Water
Salt (to taste)
1 tbsp Turmeric
2½ tbsp ground Coriander
1 tsp ground Cumin
1 tsp Chilli powder
2 tsp Brown Sugar
2–inch piece fresh Ginger (peeled and ground to a paste)
2 small Tomatoes (chopped)
1 tbsp Vegetable Oil
2 tsp Black Mustard seeds
1 tsp whole Fenugreek
1 tsp Hing (Asafoetida)
4–6 fresh Green Chillis

1. Wash daal thoroughly and let soak in a bowl, covered in water, for 30 mins. Drain water and set daal aside. 
2. Bring 2½ quarts of water to a rolling boil in a large stockpot. Add daal and salt, and cook, covered, over medium-high heat, occasionally skimming the foam and scum that gathers on the top. When the daal is soft and the grains start to fall apart, about 30-35 mins, add turmeric, coriander, cumin, chilli powder, brown sugar, ginger and tomatoes. Continue simmering for another 25–30 mins, until the daal is cooked through. Turn heat to low and let daal simmer, covered, as the finishing spices are prepared. 
3. Heat oil in a small frying pan or skillet over high heat. When the oil is smoking, add black mustard and allow to sputter until they start to pop, a matter of seconds. Add fenugreek, hing, and green chillies and fry for 1–2 mins, until lightly golden. Remove from heat. 
4. Add fried spice mixture to cooked daal and simmer, covered, for 10 minutes. Serve hot with rice or naan.