1. Place fish pieces and lime juice in a covered plastic or glass container in the refrigerator. Allow to marinate overnight (or for at least 8 hours), stirring several times, if necessary, to make sure all of the fish is covered.
2. When fish turns white/opaque, drain off and discard the marinade. Combine the fish with the tomato, cucumber, shallot, chile pepper, cilantro and olive oil in a medium bowl.
3. Whisk rice vinegar with fish sauce and brown sugar and pour over the fish. Add a pinch or two of red pepper flakes. Mix well and chill for 1-2 more hours before serving, garnished with a bit more red pepper flakes, if desired.