Heat oven to 350° degrees and position racks in top and bottom thirds. Wash potato (or potatoes) thoroughly with a stiff brush and cold running water. Dry, then using a standard fork poke 8 to 12 deep holes all over the spud so that moisture can escape during cooking. Place in a bowl and coat lightly with oil. Sprinkle with kosher salt and place potato directly on the rack in the middle of the oven. Place a baking sheet on the lower rack to catch any drippings.
Bake 1 hour or until skin feels crisp but the flesh beneath feels soft.
When there is about 20 minutes left on the potatoes, heat oil over medium-high heat in a large skillet. Cook the ground beef until browned, then drain the excess grease. Stir in taco seasonings and water. Bring to boil. Reduce heat and simmer 5 minutes, stirring occasionally.
With a sharp knife, cut an X in the top of each fresh, hot baked potato. Fluff pulp with a fork.
Place each potato on a cookie sheet and top each potato with taco meat and cheese. Broil in the oven on low for 2-5 minutes to melt the cheese. Top with green onions.