Peel the potatoes and cut into roughly 1-inch cubes. Place in a medium pot with water, two teaspoons of salt and cover.
Bring to a boil, then simmer for about 12-16 minutes or until the potatoes are tender.
Drain the potatoes and return them to the pot.
Add the butter, additional 2 teaspoons of salt, fajita taco spice and queso fresco to the cooked potatoes. Using a potato masher, mash the mixture until smooth.
Warm the tortillas one at a time on both sides in a dry 12-inch frying pan. While the tortilla is warm, spread about a quarter cup of potato into each tortilla.
Place a thin slice of thinly sliced tomato and gently fold in half to form a taco.
When all tacos are assembled, heat the oil in the same frying pan until hot and shimmering.
Fry each taco on one side until crisp, about a minute and half, then flip with tongs and repeat. You can fit 3 in the pan at a time.
Drain tacos on a paper towel and sprinkle with a little additional salt if you like.
To serve, add sour cream, guacamole, or your favorite salsa.
