Put the halved potatoes in a saucepan and cover with cold water by about an inch. Salt the water and bring to a boil. Lower the heat and simmer gently for about 15–20 minutes, until a knife slides into the centers easily but the potatoes still hold together. Roast the peppers while the potatoes cook.
Heat the broiler to high. Put the whole poblanos on a rimmed baking sheet about 6 inches from the heat. Broil, turning with tongs and watching closely, until the skins are blistered and blackened in patches, about 10–15 minutes. Transfer to a bowl, cover, and let stand for about 10 minutes. Once cool enough to handle, rub off the loose skins, remove the stems and seeds, and roughly chop the peppers.
Drain the potatoes well and let them cool just enough to handle. Peel and cut into roughly 1/4-inch dice. Allow any remaining surface moisture to evaporate before frying.
Heat the 3 tablespoons of olive oil in a wide, heavy skillet over medium heat. Add the potatoes and sliced onion. Cook for about 12–15 minutes, gently turning sections with a spatula, until the potatoes have golden-brown patches and the onion is soft. Give the potatoes time against the pan between turns. Lower the heat if the onion starts to darken too quickly; use two batches if needed.
Stir in the chopped poblanos and optional epazote. Cook for about 2 minutes, until hot throughout. Take the skillet off the heat and gently fold in the crumbled queso fresco. Taste, then add salt as needed. The cheese should stay in soft crumbles.
Use the remaining 2 tablespoons of oil as needed to keep a separate skillet lightly coated over medium heat. Warm each tortilla briefly on both sides until hot, flexible, and lightly spotted. Keep the warmed tortillas under a clean towel. Do not fry them until stiff.
Divide the warm potato filling among the tortillas and fold them in half. Serve right away, with hot sauce or salsa on the side if you like. Makes about 12 tacos, or 4 servings of 3 tacos.