- Process first 6 ingredients in the container of an electric blender or food processor until smooth, set aside.
- Combine water and next 4 ingredients in a large skillet; bring to a boil.
- Cover, reduce heat, and simmer 5 minutes.
- Add fillet; cover and simmer 20 minutes or until flakes easily when tested with a fork.
- Gently remove fillet, and discard liquid.
- Cover and chill fillet 1 hour.
- Flake fillet with a fork; arrange fillet, carrot, and tomato on a lettuce-lined serving platter.
- Serve with horseradish dressing.