Heat the oil in a large saucepan over high flame. Add the onion and garlic scapes. Cook and stir until the onion softens, about 5 minutes. Add the sweet potato, ginger, garlic, curry powder, bay leaf and summer savory; cook and stir another few minutes. Pour in the hot water and the lentils. Reduce the heat to medium-low, cover and simmer until the lentils break down and the sweet potatoes are tender, 20 minutes or so. While the potatoes and lentils are cooking, wash the radish leaves and trim away the stems, then chop coarsely. Remove the bay leaf. Add the radish leaves about 5 minutes before serving. Season with salt and a pinch of cayenne pepper.