First, roast the poblano peppers. You can either place them on a baking sheet under the broiler until the skin is charred, or you can place them directly over a gas burner.
Either way, rotate peppers to char as much of the skin as possible. Then place in a covered bowl for 5 minutes to let the peppers sweat off some of the skin.
Next, heat the olive oil in a soup pot. Add the onions, season with salt and pepper, and saute for 3 minutes, stirring frequently. Then add the garlic.