Prep the jicama: Slice off the top and bottom of the jicama to create flat surfaces. Use a sharp knife to cut away the thick, papery skin. Cut into 1/2-inch-thick sticks.
Rinse and pat dry. Set aside.
Make the guacamole base: Halve the avocados, remove pits, and scoop the flesh into a medium bowl. Add lime juice and mash with a fork until mostly smooth with some small chunks.
Add mix-ins: Stir in red onion, jalapeño, garlic, cilantro, cumin, salt, and pepper.
If using tomato, fold it in gently so it doesn’t water down the guacamole.
Adjust seasoning: Taste and add more lime, salt, or pepper as needed. For a smoky note, sprinkle in a pinch of chili powder or smoked paprika.
Serve: Pile the jicama sticks on a platter. Transfer the guacamole to a bowl and set it in the center.
Serve immediately for peak freshness.