In a large pan or kadai, heat oil. Add mustard seeds and once it pops add cumin seeds, fennel seeds, asafoetida and fry till golden.
Add the curry leaves, garlic, ginger, onion and fry till the onion starts getting golden.
Add the tomato puree, turmeric powder, red chilly flakes, sambar powder, pepper powder and fenugreek powder. Cover and cook for 10 minutes stirring in between.
Add 1 cup water, salt and cover and cook again for 10-12 minutes or till the gravy has thickened stirring in between.
Add the beans, coconut milk, 1 cup water, adjust the salt and mix well. Cover and cook for another 10 minutes.
Remove from heat and add the lemon juice and coriander leaves.