1 1/2 cups dry yellow lentils
2 1/2 cups water
1/4 teaspoon ground turmeric
1 sweet potato, peeled and cut into 1-inch cubes
2 tablespoons vegetable oil, divided
1 onion, finely chopped
1 tomato, finely chopped
3 fresh jalapeno peppers, seeded and finely diced
salt and freshly ground black pepper to taste
chopped fresh cilantro, for garnish