Prepare the Avocados: Slice the avocados in half lengthwise and twist to separate. Carefully strike the pit with the blade of a sharp knife so it lodges, then twist the knife to remove the pit. Scoop the avocado flesh into a medium-sized mixing bowl.
Mash to Perfection: Immediately add the fresh lime juice to the bowl with the avocados. This helps prevent browning while you prepare the other ingredients. Using a fork or a potato masher, gently mash the avocados to your desired consistency. For a chunkier texture, leave some larger pieces. For a smoother dip, mash more thoroughly.
Add the Mix-Ins: Add the finely diced white onion, chopped tomato, minced garlic, and fresh cilantro to the bowl.
Combine and Season: Gently stir everything together until just combined. Be careful not to overmix, as this can make the guacamole pasty. Season with salt and pepper.
Taste and Adjust: This is the most important step. Taste the guacamole with a chip and adjust the seasonings as needed. It might need more salt to enhance the flavors, more lime juice for brightness, or more onion for a sharper bite. Serve immediately.