Preheat broiler.

Spread the chiles on a baking sheet. Place under the broiler, about 5 inches from the heat and broil until the skins are charred. Turn the chiles and repeat.

Remove the chiles and place them in a bowl. Cover with plastic wrap or a lid so they can steam for 15 minutes. 

After 15 minutes, remove the skins by wiping with paper towel. Remove the stem and seeds and chop.

To a sauté pan, add vegetable oil. Add the onion and sauté until starts to soften. Add garlic, cook for 1 minute.  Sprinkle in the flour, stir and cook for 1 minute. Add the stock while whisking to prevent lumps. Add the chopped chiles and salt. Bring to a boil and simmer for 15 minutes until thickened. Transfer to a bowl and allow to cool. Taste for seasoning, add more salt if you like.

Once cool, transfer to a container with a lid and refrigerate for up to 1 week.