In a large skillet over medium heat, heat oil. Add onion and cook, stirring, until soft, 5 minutes. Add ground beef and taco seasoning. Cook, breaking up meat with a wooden spoon, until no longer pink, 8 minutes. Drain fat. 

Flip over tomatoes so they’re stem-side down and slice to make 6 wedges, being careful not to cut all the way through. Carefully spread open wedges. 

Divide taco meat among tomatoes, then top each with cheese, lettuce, and sour cream before serving.