Mash avocados in a medium bowl. Add finely chopped tomato and red onion to the bowl.
Squeeze the juice of a lime, season with cumin, salt, and pepper, and mix everything. Season with more salt and pepper according to your taste.
Serve immediately, or cover with a plastic wrap to prevent air reaching it. (The oxygen in the air causes oxidation which will turn the guacamole brown.) Refrigerate until ready to serve.
Notes
Choose Ripe Avocado - The secret for perfect guacamole is to use good, ripe avocados. Choose avocados that are slightly soft with dark green skin. (Below, I am sharing my tips on how to choose a perfect avocado every time). But if you can't find ripe avocados, you can also ripen hard avocados at home in about a day. Place them in a paper bag with a couple of bananas. The ethylene gas released by the bananas will help them ripen quicker. And if you find perfectly ripe avocados but are not planning to use them right away, you can store them in the fridge to slow their ripening.
Use Roma Tomatoes - Roma tomatoes are perfect for making homemade guacamole because they are not as watery as other kinds of tomatoes. (because who wants it to be watery and mushy? I thought so - no one!)
Don't Skip the Lime Juice - Lime juice prevents the avocado dip from browning and adds a nice fresh flavor. It's essential to add some acidity- if you don't have a lime, you can also substitute it with lemon juice.
Don't Overmash the Avocados- to achieve the perfect texture and consistency, simply mash the avocados using a fork or a potato masher. Stop mashing when it is still a little chunky.
Season Well - season the avocado guacamole to your taste with salt, freshly ground black pepper and a pinch or two of my special ingredient - cumin. It adds a really nice flavor!