Roast the chiles over a gas flame or under a preheated broiler, turning often, until charred all over. Transfer to a large plate and let cool. Discard the charred skin, seeds and stems and cut the chiles into 1/2-inch dice. Transfer the chiles to a bowl and stir in the garlic. Season with salt and pepper.
The green chile relish can be refrigerated for up to 3 days.