Preheat the oven to 425°F. Place poblano peppers on a baking sheet and roast for 25-30 minutes, turning halfway through, until the skins are charred and blistered.
Remove the peppers from the oven, cover with a kitchen towel or place in a paper bag for 10 minutes to steam, then peel off the skins and remove the seeds.
In a large pot, heat olive oil over medium heat. Add chopped onion and minced garlic, sautéing until onion is translucent, about 5 minutes. Add the chopped roasted poblano peppers and cook for another 2 minutes.