For the taco filling: Boil the potatoes until al dente. Drain and mash roughly.
Season with cumin, chili powder, salt and pepper.
While the potatoes are boiling, sauté onions in a small amount of oil over medium heat. Cook until tender and golden, adding water to deglaze the pan as needed.
Add the onions to the potatoes, then stir in cheese.
In another pan, heat 2 tablespoons cooking oil over medium heat. Add corn tortillas to the oil. When the tortillas have softened, spoon potato mixture onto tortilla and fold in half. Fry tacos on both sides until crisp.
For the avocado relish: carefully combine all ingredients.
Serve tacos with avocado relish and Mexican Cesar salad.