Slice the avocados in half and remove the seeds. With a spoon, scoop out the avocado and add to a medium bowl. Mash with a fork until the consistency is how you like it. (if you like some texture, stop mashing when there are still small chunks of avocado)
Add lime juice, minced garlic, salt, onion powder, and black pepper. Stir everything together to combine. Taste and add more salt and lime juice if it seems like it needs a bit more flavor.
Spoon guacamole into the avocado shells and use a spoon to spread it evenly.