1. Combine first 4 ingredients in a large glass bowl, pressing fish down so that it is submerged in liquid. Cover and chill 3 to 4 hours, stirring occasionally, until fish is opaque in center. Drain.
2. Combine jicama, if desired, tomatoes, and remaining ingredients in a large bowl. Stir in halibut and onion. Serve with chips or tostadas.