Red lentils with cucumber yogurt

1 cup split red lentils, soaking in 1 1/2 cups water (or more)

1 bunch cilantro, chopped into stems and leaves
1 small onion, peeled
2 1/2 inches peeled fresh ginger
3 garlic cloves, peeled
1 fresh green chile

1 1/2 tsp mustard seeds
4 tbsp oil
1 1/2 tsp ground coriander
1 tsp ground cumin
1/2 tsp ground turmeric
1/4 tsp sweet paprika
10 curry leaves
1 can tomatoes (or 2 cups chopped)
2 tsp sugar
1/4 tsp ground fenugreek
pinch of asafetida

1/3 cup unsalted butter
1 1/2 tbsp lime juice
more cilantro leaves

Cucumber yogurt:
1 cup Greek yogurt
1 cup finely diced cucumber
1 1/2 tbsp olive oil

Put the cilantro stems, onion, ginger, garlic, chile in a food processor; chop them a bit but don't make them a paste.
Heat the mustard seeds on medium. When they pop, add the onion mix, cook 10 min.
Add everything from coriander to curry leaves, cook 5 min.
Add lentils, tomatoes, sugar, fenugreek, asafetida, and some salt. Probably more water too, you don't want them to dry out. Cook 30 min or until lentils are cooked.
Stir in butter, lime, and cilantro leaves if you want.
Serve with the cucumber yogurt.

Source: Plenty by Yotam Ottolenghi