Halve the avocados, remove pits, and scoop flesh into a medium mixing bowl. Finely dice the red onion to yield about 1/4 cup. Finely chop the cilantro leaves (discard thick stems) to yield about 1/4 cup. Mince the garlic clove. If using tomato, remove seeds and dice small (about 1/2 medium tomato).

Prepare jalapeño

Slice the jalapeño in half lengthwise, remove seeds and membranes for milder heat (reserve some seeds for more heat), then finely mince. Use gloves or wash hands after handling.

Mash and combine

Add the lime juice and kosher salt to the avocado in the bowl. Using a fork or potato masher, mash the avocado to your preferred consistency — smooth with some small chunks (about 8–10 gentle mashes). Stir in the minced garlic, diced red onion, chopped cilantro, and minced jalapeño until evenly distributed.

Season and finish

Taste and adjust seasoning: add more salt (up to 1/2 tsp total) or additional lime juice if brighter acidity is needed. Stir in ground black pepper and optional 1 tsp olive oil for a silkier mouthfeel. If using tomato, gently fold in the diced tomato at the end to avoid making the guacamole too watery.

Serve

Transfer guacamole to a serving bowl. Press plastic wrap directly onto the surface to minimize browning if not serving immediately. Serve at room temperature with tortilla chips or as desired. Best within 1–2 hours for peak color and texture.