Prepare the vegetables by washing the peppers and tomatillos and roughly chopping the garlic.
Heat a cast-iron skillet over medium-high heat and add the peppers and tomatillos. Everything should touch the bottom of the skillet. Dry roast for 5-7 minutes on each side without stirring or flipping. The goal is have large portions of the peppers and tomatillos charred black.
Remove the vegetables from the skillet and allow them to cool. Core the peppers and seed the poblanos. Add everything to a blender along with the chopped garlic. Blend until a thick sauce forms.
In a large pan, melt the butter over medium-low heat. Add flour, cumin, and salt and whisk into a thick paste. Whisk constantly for 1-2 minutes to cook the flour taste out of the roux.
Add the broth, blended peppers, and sour cream to the roux. Mix everything together and increase the heat to medium-high. Once the sauce is at a low simmer, turn the heat to medium and cook the poblano crema for 4-5 minutes or until the sour cream is well-incorporated and the sauce is hot throughout.
