Start by roasting the poblanos. I like to put the peppers directly over the flame of the gas burners, rotating the peppers until all of the skin is charred. Then I place the peppers in a covered bowl for 15 minutes. This lets the peppers sweat off the skin a bit, and gives them time to cool. After 15 minutes peel off the skin, and loosely chop the peppers. I recommend discarding the seeds, unless you want an extremely spicy sauce.

Next, melt the butter in a saucepan over medium heat. Then add the garlic and onions. Cook until the onions are translucent. Then add the chicken broth and heavy cream. Bring to a boil, add the poblanos, and let boil and reduce for 6 minutes.

Pour the poblano mixture into a blender and liquify. Season with salt, pepper, and cumin, and add ½ cup sour cream. Blend to combine. Add more sour cream and salt to taste.