Prepare the Potatoes. Start by peeling and dicing the potatoes. Boil them in salted water until they are tender, about 10-15 minutes. Drain and set aside.
Make the Filling. In a skillet, heat olive oil over medium heat. Add the diced onion and cook until translucent. Add the minced garlic and cook for another minute. Add the cooked potatoes, salt, black pepper, and ground cumin. Mash the potatoes and mix until everything is well combined and heated through.
Assemble the Tacos. Heat the corn tortillas in a dry skillet or microwave until pliable. Spoon a portion of the potato filling onto each tortilla and fold in half.
Fry the Tacos. In a large skillet, heat vegetable oil over medium-high heat. Fry the filled tacos in batches until golden brown and crispy on both sides, about 2-3 minutes per side. Drain on paper towels.
Garnish and Serve. Transfer the fried tacos to a serving platter. Top with shredded lettuce, diced tomatoes, avocado slices, salsa, and sour cream. Serve immediately.