This buttery textured parfait of tuna and salmon makes a refreshing starter for any special meal. The savory pearls of caviar topped with a sliver of creme fraiche combine elegantly with the piquant flavoured fish. It goes together very easily and the end result is very impressive and tasty! If you can't get mustard oil in your area, a good substitute is 2 TBS olive oil mixed with 1/2 TBS dry mustard powder. Cook time is chill time.