Cover the beans in cold water and allow them to soak for at least 3 hours.
Place the beans in a 20cm or 8" saucepan, sprinkle over the turmeric, add the bay leaf, and pour over 1 litre (4 cups) of water.Turn the high to heat, bring the water to a boil, then reduce the heat to medium-low and simmer for 60-90 minutes or until the beans have softened.
Cut the top off the onion, chop it in half, peel it, and then slice it into 2-3mm (⅛") thick half-moon shapes.
Make a lengthways cut in the chilli peppers, leaving the stem in place so that they do not fall in half.
Peel and grate the ginger.
Peel and mash the garlic.
When the beans are cooked, drain them, reserving 125-175ml (½-¾ Cup) of the cooking liquid.
Heat the oil in a 20cm or 8" saucepan over a medium-high heat. Add the mustard, fennel and onion seeds, along with the asafoetida, slit chillies and curry leaves to the oil.
Stir and as soon as the seeds start popping, add the sliced onions, reduce the heat to medium-low and then cook for 10 minutes, stirring occasionally. The idea is to sweat down the onions and not to add any colour.
Add the garlic and ginger to the onions and cook for 60 seconds.
Throw in the chilli powder, coriander and garam masala powders and stir to coat.
Pour in the tomato passata and the reserved cooking water from the beans, then season with salt and sugar to taste.
Add the beans, pop on a lid and simmer for 20 minutes.
Remove the lid, crush in the dried fenugreek leaves and stir just before serving.