Arrange the poblano peppers on a large baking sheet lined with foil, so that they are spaced apart.
Broil the chiles until their skin is charred (bubbly with dark brown spots), about 5 minutes per side.
Move the chiles to one side of the pan, fold the foil over them and seal. (You can also place them in a sealed container.) Allow them to rest for 15 minutes.
Peel the skin off of the chiles.
To cut an opening into each chile: Slice down lengthwise, starting from the stem of the chile to the end. Make one more incision, perpindicular to and at the top of the first cut, just across the front of the chile. (The slices will look like a "T", see photos in the post or watch the video for a visual.)
Remove the seeds from the chiles, using your hands or a spoon to scoop them out. (I recommend wearing gloves if using your hands.)
Place the chiles in a lightly oiled or sprayed casserole baking dish, with the cut side facing up. Set aside.