Put the lentils in a pot and boil for 15-20 minutes, until very well done and mushy. Drain. Meanwhile, heat a good drizzle of oil and sauté the onions until dark golden. Add the tomatoes and garlic and cook for another 5 minutes. Add the curry paste, sugar, chili powder, and salt, then the lentils and cook for another 5 minutes. Add the half & half and cook until it has the consistency you want - they can be runny, or thicken up if you cook it for a few more minutes.
Remove from heat and stir in the garam masala, then sprinkle with cilantro, if you like.