Heat the oil in a skillet and add the onions and garlic to it. Give it a quick stir and add the potatoes to it. Keep it on medium low heat and cover and cook till the potatoes are just soft.
Add the ancho chili, cumin and paprika and mix it well. Add salt as well.
On an open flame, roast the poblano till charred on the surface. Cut it in half, discard seeds and chop the rest. Add this to the potato mixture.
Heat the tortillas in a skillet until soft and give it a quick char directly over the flame (optional).
To assemble, stack two tortillas together and place some lettuce leaves on it. Top with the potato poblano mixture and then some of the onions and cotija cheese. Drizzle some sour cream over it. Garnish with cilantro and squeeze a little of the lime over it.Repeat with the rest of the tortillas.