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.
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.
Pour the poblano mixture into a blender and liquify. Season with salt, pepper, and cumin, and add 1/2 cup sour cream. Blend to combine. Add more sour cream and salt to taste.