Make the filling: In a large pot or Dutch oven, combine the potatoes, garlic, and water to cover by about 1 inch. Bring to a boil over high heat, then reduce the heat to medium-low and simmer until fork tender, about 15-18 minutes.
While the potatoes cook, place the longaniza or chorizo in a medium skillet over medium heat and fry for about 10-12 minutes, until the fat has rendered and the sausage is crispy. Use a slotted spoon to transfer the sausage over to a plate lined with paper towels to drain.
Once the potatoes are done, transfer them and garlic to a bowl, add the salt and pepper, and mash. Fold in the chorizo and adjust the salt to taste.
To assemble: Working in batches of 3 to 4, pop the tortillas in the microwave for about 45 seconds, or heat them up on a comal over medium heat for about 1 minute on each side. (Cold tortillas will rip when folded.)
In a large skillet, heat about a cup of vegetable oil over medium heat. Add 2 to 3 tablespoons of the mashed potatoes to a tortilla, fold the tortilla in half, and fry until crispy and golden brown on both sides, 2 to 3 minutes per side.
To serve, place a couple of tacos on a plate then top with shredded lettuce (or cabbage), your favorite salsa, diced onion, sliced radishes, crumbled cotija and slices of avocado.