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.