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1
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.
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2
Transfer the roasted poblanos to a food processor and puree until smooth.
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3
Heat the butter in a medium skillet over medium-high heat.
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4
Add the onion, reduce the heat to medium, and cook, stirring often, until the onion is soft and golden, 4 to 5 minutes.
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5
Transfer the onion to a plate and set aside to cool.
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6
Bring a medium saucepan of water to a simmer.
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7
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.
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8
Drain the tomatillos in a fine-mesh sieve then run them under cold water to cool.
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9
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.
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10
Bring the vegetable broth to a simmer in a saucepan.
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11
Add the poblano-tomatillo puree, cream and sour cream and reduce the heat to medium-low.
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12
Gently simmer the sauce, stirring often, until light and creamy, 45 minutes to 1 hour.
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13
Use an immersion blender to blend until smooth (or blend the sauce in batches using a standard blender).
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14
The sauce can be refrigerated for up to 4 days or frozen for up to 4 months.
