Preheat oven to 475 degrees. Scrub potatoes and prick several times with a fork. Bake for 60 to 75 minutes, until soft.
Meanwhile, cook ground beef and onion in a nonstick pan until beef is cooked through. Break it into small pieces with a wooden spoon as it cooks.
Add taco seasoning and 1/4 cup water. Cook for 1 minute. Remove from heat.
Once potatoes are baked, let cool 10-15 minutes and then remove a sliver of skin from the top of each potato. Scoop potato pulp out and place in a large bowl. Add sour cream and milk and mash until smooth. Stir in about 3/4 of taco mixture. If you feel like there will be too much meat mixed in if you add the remaining 1/4, save it to make a quick quesadilla or tortilla pizza for lunch. Or go ahead and mix it in. Fold in 3/4 cup of cheddar cheese.
Taste for seasoning. Taco seasoning tends to be salty so you may not need to add much, if any salt.
Stuff the potato mixture back into the skins. Place in a baking dish. Sprinkle remaining cheese on top and bake at 350 for 20 to 25 minutes.
To serve top with green onions, salsa, and sour cream.