Wash the lentils in cold water until the water runs clear. Put lentils in a pot with enough water to cover; bring to a boil, place a cover on the pot, reduce heat to medium-low, and simmer, adding water during cooking as needed to keep covered, until tender, 15 to 20 minutes. Drain.
Heat vegetable oil in a large skillet over medium heat; cook and stir onions in hot oil until caramelized, about 20 minutes.
Mix curry paste, curry powder, turmeric, cumin, chili powder, salt, sugar, garlic, and ginger together in a large bowl; stir into the onions. Increase heat to high and cook, stirring constantly, until fragrant, 1 to 2 minutes.
Stir in the tomato puree, remove from heat and stir into the lentils.
