Preheat the oven to 425 degrees F. Place the poblano halves on a baking sheet and roast until they begin to shrivel, about 20 minutes. Transfer the roasted poblanos to a food processor and puree until smooth.
Heat the butter in a medium skillet over medium-high heat. Add the onion, reduce the heat to medium, and cook, stirring often, until the onion is soft and golden, 4 to 5 minutes. Transfer the onion to a plate and set aside to cool.
Bring a medium saucepan of water to a simmer. Add the tomatillos and blanch them until they turn vibrant in color and the center of the largest one is easily pierced with a fork, about 5 minutes. Drain the tomatillos in a fine-mesh sieve then run them under cold water to cool. Transfer the blanched tomatillos to the food processor bowl with the poblano puree, add the cooled onion, pears, green chiles, cilantro, and white pepper and process until smooth.
Bring the vegetable broth to a simmer in a saucepan. Add the poblano-tomatillo puree, cream and sour cream and reduce the heat to medium-low. Gently simmer the sauce, stirring often, until light and creamy, 45 minutes to 1 hour.
Use an immersion blender to blend until smooth (or blend the sauce in batches using a standard blender). The sauce can be refrigerated for up to 4 days or frozen for up to 4 months.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.