Cook the lentils according to directions. Drain and set aside.
Melt the butter in a large saucepan over medium high heat. Add the onion and saute for a few minutes until fragrant and golden. Add all the spices (curry paste, garam masala, curry powder, turmeric, cayenne, sugar, garlic, ginger) and stir fry for 1-2 minutes. Add the tomato sauce; stir and simmer until smooth.
Add the lentils and the cream. Stir to combine and simmer for another 15-20 minutes (the longer, the better)! Serve over rice and garnish with cilantro.
Even better the next day!
For a vegan option, sub oil for butter. from http://pinchofyum.com/red-curry-lentils