Despite the smoky depth that comes from dried roasted chiles, this broth dances lightly on the palate -- a restorative in its own right.
Preheat broiler. Place tomato, serrano chile, and garlic on a rimmed baking sheet. Drizzle with 2 teaspoons oil, and broil, turning twice, until tomato has softened and chile and garlic have charred, 5 to 8 minutes. Let cool, then finely chop.
Heat remaining 2 teaspoons oil in a medium saucepan over medium-high heat. Add tomato mixture and pasilla chile, and cook, stirring often, for 1 minute. Stir in stock and 1 1/2 teaspoons salt. Bring to a boil, and cover. Reduce heat, and simmer for 10 minutes. Discard pasilla chile.
Stir in lime juice, cilantro, and onion. Season with salt. Serve immediately.
This light soup calls for turkey stock but not the meat. You can, of course, add some to create a more substantial dish.