In a large, heavy skillet, heat a drizzle of oil over medium-high heat. Saute the onion for about 5 minutes, then add the jalapeño, ginger, garlic, curry paste, cumin, turmeric and salt. Cook for a few more minutes.
Stir in the sweet potato, lentils, coconut milk and water; bring to a simmer, then cover, turn the heat down to medium-low and cook for about 20 minutes, until the potatoes are tender.
Uncover and tear in the spinach; stir, return the lid and cook for another minute or two, just until the spinach wilts. Serve immediately, over rice.