Preheat the oven to 475°F (245°C). Scrub the potatoes, pierce them several times with a fork, and bake directly on the oven rack or a baking sheet until completely tender, about 60–75 minutes.

While the potatoes roast, heat a nonstick skillet over medium-high heat. Add the ground beef and chopped onion, breaking the meat into small pieces as it browns, and cook until no pink remains.

Stir the taco seasoning and 1/4 cup water into the cooked beef, simmer for about a minute to blend flavors, then remove the pan from the heat and set aside.

When the potatoes are done, let them cool for 10–15 minutes so they’re easier to handle. Trim a thin slice off the top of each potato, scoop the flesh into a large bowl, and reserve the empty skins.

Add the sour cream and milk to the potato flesh and mash until smooth and creamy. Fold in about three-quarters of the seasoned beef mixture and three-quarters of the shredded cheddar; reserve the rest of the meat (or use it for another meal) and cheese for topping.

Taste the filling and adjust seasoning if needed—taco mixes can be salty, so add salt sparingly if required.

Spoon the mashed potato and beef mixture back into the reserved potato skins, arrange them in a baking dish, sprinkle the remaining cheddar over each, and bake at 350°F (175°C) until the cheese melts and tops are lightly browned, about 20–25 minutes.

Serve hot topped with sliced green onions, salsa and additional sour cream as desired.