Add the red onion, lime juice, garlic, and salt to a large bowl.
Halve, pit, and scoop the avocado flesh out of the skins and place it in the bowl with the onions. Mash gently with a fork until the avocado is as smooth or chunky as you prefer.
Add the tomatoes, cilantro, and jalapeño, and stir together until well combined. Taste and add more lime juice or salt, if desired. Serve immediately.
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Notes
You can use a fork or potato masher to mash the avocados to the consistency you like (chunky or smooth). The potato masher is great if you want a really smooth dip. Use a fork for chunkier guacamole.
If your avocados are on the small side and you don't seem to have enough, you can add a little sour cream to bulk it up.
Marinate the onions and garlic with the lime juice first! This takes a little bit of the bite out of the fresh onions and imparts a great mellow flavor. Simply mix the onions, garlic, lime juice, and salt together in the bowl and let them sit for 10 minutes before adding in the avocado and mashing.