Preheat the oven to 400 degrees F. Wash the sweet potatoes well and pat them dry. Poke each sweet potato several times with a fork and wrap in foil. Place sweet potatoes on a baking sheet. Bake 60 to 90 minutes (depending on size of sweet potatoes), until potatoes are very tender.
While the sweet potatoes are roasting, you can prepare the taco filling. Add oil to a large cast iron skillet and heat to medium. Add the onion and saute, stirring occasionally, until onion has softened and is translucent, about 5 to 8 minutes.
Scoot the onion off to the side of the skillet and add the ground beef, leaving it in a layer (don't break it apart quite yet). Brown the meat 2 minutes, then flip to the other side and brown another 2 minutes.
Add the garlic, chili powder, and sea salt, and stir everything together well. Cook for 2 minutes, until the garlic is very fragrant.
Add the chopped tomato, cover, and cook 10 minutes, stirring occasionally.
Remove cover and cook another 5 to 8 minutes, stirring occasionally, until much of the liquid has burned off.
Slice the roasted sweet potatoes in half length-wise and scoop out some of the flesh (leave about 1/2 inch of flesh around the skin), saving the flesh for muffins, pancakes, or mashed sweet potatoes.
Stuff the sweet potatoes with desired amount of taco meat. Sprinkle with cheese and chives and serve. Note: you can heat the stuffed sweet potatoes in the oven for a couple of minutes to melt the cheese if desired.