Scrub the potatoes until clean. Prick them evenly all over the surface with a fork. Place them on a baking sheet and bake at 475 degrees for an hour, or until soft. Depending on your oven and altitude, it could take longer.
White the potatoes are cooking, add the beef & onion to a large skillet and saute-stirring and breaking up- over medium heat until the beef's cooked through. Strain, and return to the skillet.
Stir the taco seasoning and water into the meat until evenly incorporated. Continuing cooking over medium heat, just until the sauce has thickened. Remove from heat, set aside.
Remove the cooked, soft potatoes from the oven. Transfer them to wire racks and let them cool for about 15 minutes. Use a paring knife to remove just the top bit of skin from the potato, creating a 'lid'.
Using a spoon, scoop the soft flesh out of the center of the potato, and transfer to a large mixing bowl, to create a bowl. Scoop out as much as you can, just leaving a thin layer intact to prevent leakage. Repeat for all potatoes.
Add the sour cream, half & half, taco meat mixture, and 3/4 cup of cheese to the mixing bowl. Stir/mash everything together until the mixture's smooth and evenly incorporated. Salt & pepper, to taste.
Scoop the mixture back into the hollowed out potatoes, evenly, until they're all completely stuffed. It's ok if it goes above the rim, and also ok if you need to pack them down some to fit it all. Return the stuffed potatoes to the baking sheet.
Sprinkle the remaining 1/4 cup of cheese evenly out over top of the potatoes. Bake at 375 degrees for 15-20 minutes, watching so the cheese melts and crisps but doesn't burn. Remove the potatoes from the oven, again transferring them to cooling racks, and let them cook for 5-10 minutes.
To serve, top them with a dollop of sour cream, salsa, and a sprinkle of green onion.