Preheat the oven to 350 degrees . Place the potato in a saucepan and add enough water to cover by 1 inch. Bring to a boil over high heat. Lower the heat and simmer until very tender, about 10 minutes; drain.
Meanwhile, in a large skillet, cook the beef over medium heat, breaking up the meat, until cooked through, about 5 minutes. Using a slotted spoon, transfer the beef to a bowl. Add the potato, mashing slightly and stirring to combine; season with salt and pepper.
Wrap the tortillas in damp paper towels, then in foil. Place in the oven until warm, about 5 minutes. Meanwhile, in another large skillet, heat enough oil to reach a depth of 1/2 inch over medium-high heat. Working in batches, and keeping the rest of the tortillas wrapped, place about 1/3 cup of the beef mixture on one side of the tortilla and fold into a half-moon. Place in the skillet and fry, turning once and gently pressing the tortilla to ensure that the taco holds its shape, until golden, about 5 minutes. Transfer to paper towels to drain. Gently crack open each taco and fill with the lettuce, tomatoes, guacamole, salsa and sour cream.