Cut the avocados in half. Remove the pit and scoop out a bit of avocado to create a larger well which will be filled with the beef mixture. Squeeze a little bit of lime or lemon juice over the open avocados to prevent browning.
In a medium pan, heat oil over medium heat and saute onion until soft (about 5 minutes). Add ground beef and fry until brown.
Add taco seasoning and water and cook over low heat for 5 minutes.
Season with salt and pepper as needed.
Remove from heat and let cool a little.
Fill the holes in each avocado with beef, then top with cheese, tomato, and a little sour cream.
