Wash and soak the red lentils.
Grind the fenugreek, black pepper, cayenne/red chilies, turmeric into a powder and keep aside.
In a saucepan, heat oil over medium heat. When hot, add the mustard seeds and let them start to pop. Add curry leaves carefully and mix in. Add the fenugreek seed powder, stir and cook for 5 to 10 seconds.
Add onions, cinnamon stick and a pinch of salt. Mix to coat with spices. Cook for about 3 minutes.
Drain and add the lentils to the saucepan. Add water, salt and coconut milk. Mix in. Partially cover and cook for 12 to 14 minutes.
Add in the tomatoes, lemon juice, taste and adjust salt and spice. Lower the heat to low. Cover and cook for 12 to 15 minutes. Stir once in between.
Garnish with shredded coconut and pepper flakes (optional). Serve as a soup with crackers or as dal with rice or flatbread.
