Drain the sour cream overnight in a cheesecloth-lined sieve, mix dill into sour cream. Coat the inside of the pastry ring with a thin coating of vegetable oil and place the pastry ring in the center of a decorative serving platter.
Fill the bottom of the ring with smoked salmon that is thinly sliced and layer it into one even layer. Spoon a thin layer of dill cream onto the salmon. Top with an even layer of salmon caviar, being careful not to break any of the eggs. Top with dill cream. Finish with a layer of black caviar and dollop with dill cream. Chill in ring for 1 hour or more before serving to allow parfait to set. Remove the ring carefully and garnish the top of the parfait with sprigs of dill. Use watercress leaves and crisp bread to garnish the platter.