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Great guacamole starts with properly ripe avocados — the flesh should yield to gentle pressure without feeling mushy. Hass avocados are the standard choice for their buttery fat content and deep flavor. If your avocados are firm, leave them at room temperature for one to two days.
Cut each avocado in half, remove the pit, and scoop the flesh into a bowl. Add fresh lime juice immediately — this is both a flavor component and the key to preventing oxidation. Use a fork or a molcajete to mash to your preferred texture, leaving some chunks for body.
Season generously with salt before adding any other ingredients; this brings out the avocado's natural flavor. Fold in two tablespoons of finely diced white onion, a small handful of roughly chopped fresh cilantro, and half a seeded jalapeño minced fine. Taste and adjust lime and salt.
Optional add-ins that improve without overpowering: a pinch of cumin, one minced garlic clove, or a quarter cup of diced tomato added at the very end. Serve immediately for best color, or press plastic wrap directly against the surface to minimize air contact if making ahead.