Choose the right avocado. Select an avocado with a nice shade of dark green, with just a touch of firmness, but yielding slightly to gentle pressure.

The avocado should not have dark brown spots or blemishes, and avoid squeezing it to prevent bruises, which appear as brown spots inside the fruit.

Safely slice the avocado in half around the seed, twist to separate the halves, and remove the seed.

If there's a bruised section, simply scoop it out with a spoon and discard it.

Dice the avocado while it's still in the skin by making cross-hatch cuts, be careful not to pierce the skin, and scoop out the diced flesh.

Dice onions, serranos, cilantro, and tomatoes—keeping them separate at this stage. The onions should be finely diced, serranos can have seeds if you want extra heat, and dice the tomatoes last to prevent them from making the guacamole watery.

In a molcajete or a mortar and pestle (or a bowl if you don't have one), combine diced onions, minced serranos (with seeds if desired for extra heat), cilantro, and a squeeze of fresh lime juice. Mash these ingredients together to create a flavorful base.

Add the diced avocado to the molcajete and gently combine with the base, adding salt to taste, and mash to your desired consistency. Avoid over mashing; guacamole should be slightly chunky and creamy.

Stir in the diced tomatoes gently, as they can release water if overmixed. Add them last to maintain their chunkiness.

Taste and adjust the seasoning if necessary, adding more lime or salt as needed. Serve the guacamole in the molcajete (if available), which will keep it cooler. Optionally garnish with queso fresco and cilantro, accompanied by sturdy corn tortilla chips for dipping.