In a large saucepan over low heat, combine lentils, onions, curry powder, salt, turmeric, and 2 cups 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 jalapeno peppers and simmer, covered, until the cauliflower is tender, 8 to 10 minutes longer. Remove from heat.

Heat oil in a small skillet over medium-high heat. Add cumin seeds and cook for about 10 seconds. Add garlic and ginger; 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.

Serve over rice.