Cut off the corn kernels; set aside. Combine the cobs, 1 cup potatoes, 1/2 cup scallions, 1 tablespoon ginger, the garlic, peppercorns and 5 cups water in a pot. Smash the lemongrass, if using, and add to the pot. Bring to a boil, then simmer for 25 to 30 minutes.
About 10 minutes before the broth is finished, melt the butter over medium-high heat in a separate pot. Add the remaining 1 cup potatoes, season with salt and cook until slightly soft, 5 minutes. Add the remaining 1 tablespoon ginger and the jalapeno; cook 1 minute. Add the corn kernels; cook until the vegetables are just tender, 3 to 4 minutes.
Strain the broth, pressing out as much liquid as possible; discard the solids. Add 2 cups of the strained broth to the potatoes and corn; bring to a boil, cover and simmer for 10 minutes. Add the coconut milk, basil and mint; season with salt. Stir until simmering. Remove from the heat and add the radishes, cilantro and lime juice. Top with diced tomato and the remaining 1/4 cup scallions and serve with lime wedges.
Per serving: Calories 407; Fat 32 g (Saturated 25 g); Cholesterol 30 mg