Arrange the racks in your oven so that one is in the middle and one is underneath in the bottom half. Preheat the oven to 400 degrees and line a sheet pan with aluminum foil. Use a fork to poke holes around the surface of your potatoes, about 6-8 times each. Place the sheet pan on the bottom rack and position the potatoes directly on the rack above it (this way if there are any drippings they will fall on the lined sheet pan and not your oven). Bake for 45 minutes or until cooked through. Remove the potatoes from the oven and set aside to cool. Remove the lined sheet pan from the oven as well for later use. Keep the oven on at 400 degrees.
Bring a large skillet to med-high heat. Add the ground beef and cook, breaking into small chunks with a spatula or cooking spoon. Once the meat has started to brown and break into chunks, add the chopped onions and stir to combine. Cook, stirring occasionally, for about 5 minutes or until the meat is browned and the onions are softened and translucent. Add the taco seasoning and water and stir to combine. Once bubbling, reduce the heat to med-low and simmer for 3-4 minutes.
When the potatoes are cooled enough to easily handle (about 10 minutes/the time it takes to cook the meat), slice a wide slit into the top of each one. Peel the skin off of the part in the slit. Use a spoon to scoop out the insides of each potato into a mixing bowl, leaving a ¼” border around the outside of each potato shell. Line the potato shells up on the reserved sheet pan from step one.
Add the sour cream and milk to the potato flesh in the mixing bowl. Use a potato masher or a fork to mash together. Add 2 ounces of the shredded cheddar cheese and stir the cheese into the mashed potatoes. Add the taco meat from step two and stir to combine.
Spoon the mashed potato filling evenly into the slit of the potato shells and pile it high on top. Sprinkle the remaining ounce of cheddar cheese over the filling. Place the sheet pan back into the 400 degree oven and bake for about 10-15 minutes until the cheese is melted and the tops are golden. Serve warm.