Rinse lentils under running water; drain. Place lentils, 2 1/2 cups water, and turmeric in a medium pot, and bring to a boil. Cover, reduce heat, and cook until lentils are tender, about 30 minutes.
Meanwhile, place cubed sweet potato in a steamer over 1 inch of boiling water. Cover and cook until tender but still firm, about 15 minutes (time may vary depending on size of potato pieces). Once tender, mash sweet potato and lentils together; set aside.
Heat the oil in a skillet over medium heat, and cook the onion until lightly browned. Stir in tomato, jalapenos, salt, and pepper, and continue cooking another 3 to 5 minutes. Place lentils, vegetables, and potatoes in a stockpot over medium heat. Stir in water as needed to attain desired consistency. Serve warm with cilantro or green onions.
