To prepare the salmon:
In a small saucepan sprinkle gelatin over lemon juice and soften 10 minutes. In a bowl with an electric mixer beat together mascarpone and 1/2 cup sour cream until combined well. Quarter eggs and with back of a spoon force through a coarse sieve into mascarpone mixture. Add horseradish and stir well.
Add remaining 1/4 cup sour cream to gelatin mixture and heat over moderately low heat, stirring, just until gelatin is dissolved. Add gelatin mixture to mascarpone mixture and stir well.
Cut about one third salmon into strips and arrange 1 in every other ridge of a 3-cup brioche or other mold, letting ends hang over edge. Chop remaining salmon into 1/2-inch pieces and fold with chives into mascarpone mixture. Spoon mousse into mold and fold ends of salmon strips over top. Chill mousse, covered, at least 2 hours, or until firm, and up to 2 days. Run a thin knife along inside of each ridge of mold and dip mold into a bowl of hot water a few seconds. Invert a plate over mold and invert mousse onto it.
To make the pink pickled onions:
In a saucepan barely cover onions with cold water. Bring water to a boil and simmer 1 minute. Drain onions in a colander and return to pan with vinegar, salt, and cold water to just cover. Bring liquid to a boil and simmer 3 minutes. Remove pan from heat and cool onions in liquid. Pickled onions may be made 1 week ahead and chilled in liquid, covered. Makes about 2 cups.
With a slotted spoon arrange pickled onions decoratively on and around mousse and garnish with chive blossoms. Serve mousse with melba toats and/or flatbread.
Yield:about 20 servings as an hors d'oeuvre