If you can't find queso fresco top these mild tacos with shredded cheddar or Monterey Jack.
1. Preheat the oven to 400 degrees F. Dice the potatoes and onion into 1/2-inch pieces and toss with the olive oil. Spread on a foil-lined baking sheet and season with the salt, pepper, and chili powder. Roast, tossing occasionally, until the potatoes are golden brown and tender, about 25 minutes.
2. Meanwhile, char the skins of the poblano peppers on the burner of a gas stove or on a grill. Immediately transfer the charred peppers to a bowl and cover with plastic wrap; let sit 10 minutes. Gently rub the charred skin with your fingers (it should peel right off) and discard. Slice the peppers, discarding the seeds.
3. Fill the warmed tortillas with the potato-onion mixture and sliced poblano peppers. Top with queso fresco and fresh cilantro.