Preparation of Potatoes: Preheat your oven to 400 degrees Fahrenheit. Using a fork, puncture the potatoes several times. Coat each potato liberally with oil and season generously with coarse salt. Arrange the potatoes on a baking sheet and bake them, turning once halfway through, until they are tender enough to be easily pierced with a fork, approximately one hour.
Cooling and Scooping: Allow the potatoes to cool to a manageable temperature. Remove a thin slice from the top of each potato to expose the interior. Carefully scoop out the inside of each potato into a bowl, leaving about a quarter-inch of potato near the skin to maintain its structure.
Cooking the Filling: In a large skillet over medium-high heat, cook the ground beef until nearly done. Add the onions in the last few minutes of cooking, stirring until they are softened. Incorporate the taco seasoning followed by the water, and let the mixture simmer for 3-4 minutes until it thickens. Remove from heat and stir in one cup of shredded cheese along with the sour cream.
Combining and Stuffing: Mash the potato interiors that were previously scooped out. Combine these mashed potatoes with the seasoned beef mixture, mixing thoroughly. Refill the potato shells with this mixture and sprinkle the remaining cheese on top.
Final Baking: Return the stuffed potatoes to the oven and bake until the cheese on top has melted, about 5 minutes.
Serving: Serve the stuffed potatoes hot, adorned with any of the suggested toppings according to your preference.