The video tells the tale here, but they start with the old school method of roasting chilies — firing up your gas burner or BBQ and just popping the chili over it until it’s well charred on all sides. Of course, that method works well for smaller chiles, or for one or two at a time, but it’s no good if you need to roast a ton for a big pot of chili, salsa, or even to use in a marinade or huge roast.
Instead, grab a foil lined baking sheet. Smaller chilies can go on the sheet whole, but you’ll want to cut up the larger once and flatten them into a strip, much like this walkthrough shows. Flatten out the strip (or strips) on the baking sheet, and pop it into the oven under a preheated grill at the highest setting.
They will char up nicely in just a few minutes, and you’ll need to flip the whole ones from time to time so they cook evenly. Once they’re charred, pop them into a bowl and cover the bowl with plastic wrap so the skins are easy to remove with your hands, or a paper towel (although ATK correctly notes, don’t rinse them, you’ll rinse off a lot of flavour!)
Once the chilies are roasted, skinned, and the whole ones de-seeded, they’re ready to use in just about any dish. Puree them in a sauce, dice them up for chili or nachos, cut them into strips for fajitas or mixed vegetables, whatever you want.
Roast and Toast Chiles for Maximum Flavour [America’s Test Kitchen (YouTube)]