In a large saucepan over low heat, combine lentils, onions, curry powder, salt, turmeric and 2 cups of water. Bring to a simmer. Cover and cook, stirring occasionally, until the lentils are soft and the sauce has thickened, about 45 minutes.
Add tomatoes,cauliflower, and chili and simmer, covered, until the cauliflower is tender, aprox 8 to 10 minutes. Then remove from heat.
Heat oil in a small pan over medium-high heat. Add cumin seeds and cook for about 10 seconds. Add garlic and ginger. Continue to saute until the garlic is lightly browned, about 1 minute. Stir in cayenne and immediately add the oil-spice mixture to the cauliflower mixture. Stir in lemon juice, cilantro and sugar. Taste and adjust seasonings with additional salt and cayenne.