Taco Stuffed Avocados

Halve and pit avocados. Using a spoon, scoop out a bit of avocado, creating a larger well. Dice removed avocado and set aside to use later. Squeeze lime juice over all avocados (to prevent browning!). In a medium skillet over medium heat, heat oil. Add onion and cook until tender, about 5 minutes. Add ground beef and taco seasoning, breaking up the meat with a wooden spoon. Season with salt and pepper, and cook until the beef is no longer pink, about 6 minutes. Remove from heat and drain fat.  Fill each avocado half with beef, then top with reserved avocado, cheese, lettuce, tomato, and a dollop sour cream.