To roast the peppers, place them directly on the burners of a gas stove over medium heat and turn them frequently with tongs until all sides are charred black, 7 to 10 minutes. (Alternatively, the peppers can be roasted under a broiler or over a hot gas or charcoal grill.) Transfer the peppers to a plastic or paper bag, seal the bag, and let cool for about 15 minutes. Peel the peppers, remove the seeds and stems, and coarsely chop the flesh.
Roast the onion halves over medium heat, using the same procedure, until the cut surfaces are lightly charred and the onion is slightly softened, about 10 minutes. Remove from the heat and let cool. Remove and discard the root ends from the onions; coarsely chop the onions.
Heat the vegetable oil in a medium saucepan over medium-high heat. Add the pistachios, pumpkinseeds, and pine nuts and cook, stirring occasionally, until the nuts are browned. Add the peppers, onions, chili powder, garlic, cumin, coriander, salt, and pepper and stir for 1 minute. Add the chicken stock and cilantro and bring to a boil over high heat. Reduce the heat to medium low and simmer for 45 minutes. Add the chocolate and stir until melted. Add the cream, stir well, and simmer for an additional 15 minutes. Puree with an immersion blender. Serve over the sliced turkey.