Dice fish into ~1/2-inch cubes and place in a glass on other non-reactive bowl.
Add the lime juice and salt and mix.
Chop the onion and habanero very fine, wearing gloves for easy clean-up (don’t touch your face with peppery hands!), and add to the fish.
Gently toss onions and peppers with the fish and refrigerate for 3-4 hours.
Dice jicama or celery into ~1/4-inch cubes and soak in a little bit of lime juice and salt. Refrigerate.
The fish is ready when it firms up and turns opaque white.
Add jicama to fish and toss. Serve over romaine leaves.