Char the poblano chile until it blisters and blackens somewhat. I poked a long skewer into mine and held it over an open flame from the gas stove. You can grill them or broil them just as well. Put it in a small bowl or paper bag to cool, then rub the skin off with your fingers or a towel. For best taste, don't run under water to rinse the skin off. Then remove the seeds and veins, and dice the chile. Poblanos are fairly mild so the chunks can remain large.
Pan roast (in a dry pan with no oil) the onion slices and garlic with skins. Remove from heat they turn brown and soft. Remove the skins and mince the garlic, and chop the onion.
Combine all the ingredients together in a bowl, and salt to taste.