In a cast-iron skillet, heat oil over medium heat. Once the oil is hot, add potatoes. Cook and stir for 20-25 minutes until the potatoes are tender. The time will depend on the size of the potato chunks.
While the potatoes are cooking, brown the ground beef in another skillet. Drain off the fat.
Add taco seasoning and water to the ground beef and stir to coat.
Once the potatoes are cooked, top the potatoes with the ground beef, then sprinkle cheese over the beef.
Broil potatoes on high for about 3 minutes, until the cheese is melted. Watch the potatoes carefully while they are under the broiler.
Remove the potatoes from the oven and top them with pico de gallo and sour cream, if desired.