Heat the oven to 375ºF with a rack in the lower-middle position. In a large Dutch oven over medium, heat the 3 tablespoons of oil. Add the onion and ¼ teaspoon of salt. Cook, stirring frequently, until browned, 7 to 9 minutes. Add the ginger, garam masala, turmeric and cayenne; cook for 30 seconds, stirring constantly. Add the tomatoes, coconut milk, sugar, ½ teaspoon each salt and black pepper and bring to a boil, scraping up any browned bits. Reduce the heat to medium and simmer, stirring and scraping the pan, until thickened, 20 to 25 minutes.