Heat a medium nonreactive skillet over high heat until very hot. Add the corn, salt, and pepper and cook, shaking the skillet from time to time to char the corn evenly, for about 2 minutes. (The corn will pop a bit.) Add the onions, green onions, and garlic and char for 1 minute.
Add the stock and stir in the tomatoes and cilantro and bring to a boil. Reduce the heat and simmer for 15 minutes. Remove from the heat. Serve immediately, or prepare up to an hour ahead and keep warm.
Yield: 1 1/3 cups