Preheat your oven to 375 F. With a paring knife, slit one side of each potato down the middle, being careful not to cut all the way through. You want to make a "pocket" for the taco filling. Bake potatoes until they are fork-tender, about 45 minutes and remove from heat when they are done.

While the potatoes are baking, heat a large skillet to medium and add oil. Add onions and cook until the onions become translucent, about 5 minutes. Add ground beef to pan and break up with the back of a wooden spoon. Cook mixture, stirring occasionally, until meat is no longer pink, about 10 minutes. Add taco seasoning and 3/4 cup of water and let simmer until the mixture thickens, about 5 minutes. When mixture is done, remove from heat.

Take one potato and squeeze gently to open up the slit in the middle. Add a big scoop of the filling and repeat until all ingredients are used up. Serve immediately.