- Cut fish into 1/2-inch cubes; coat with 1 Tbsp. less soy sauce in microwave-safe baking dish. Cover and microwave on High 1 minute. Gently stir fish; cover and microwave 1 minute longer. Carefully remove fish and place in large plastic food storage bag; add onion.
- Combine lime juice, sugar and remaining 2 Tbsp. less sodium soy sauce; pour over fish. Press air out of bag; close top securely. Turn over several times to coat all pieces well. Refrigerate 1 hour; turn bag over occasionally.
- Pour fish and marinade into large mixing bowl; stir in oil. Add tomatoes, chilies, mushrooms and cilantro; toss gently to combine.
- Serve over avocado wedges and lettuce leaves.