Method

In a large pan over medium heat, heat oil. Add onion and cook, stirring, until soft (approx 5 minutes). Add ground beef and taco seasoning. Cook, breaking up meat with a wooden spoon, until no longer pink (approx 8 minutes). 
Cut top off tomato and remove seeds. 
Divide taco meat among tomatoes, then top each with cheese 
Bake for approx 15 minutes at 175°C