Line bottom of 1-quart soufflé dish with foil extending 4 inches beyond rim of dish on 2 sides. Oil slightly.
Soften gelatin in cold water in measuring cup. Liquefy gelatin by setting cup in pan of hot water, or in microwave oven for about 20 seconds at lowest setting. This gelatin will be divided among the 3 layers.
FOR EGG LAYER: Combine all ingredients for egg layer with 1 Tbsp of gelatin. Taste and adjust seasoning. Neatly spread egg mixture into prepared dish with spatula, smoothing top. Wipe any egg mixture from foil with paper towel.
FOR AVOCADO LAYER: Combine all ingredients for avocado layer with 1 Tbsp of gelatin. Taste and adjust seasoning. Gently spread mixture evenly over egg making sure to wipe any avocado mixture from foil with paper towel.
FOR SOUR CREAM LAYER: Mix sour cream, onion and remaining 2 Tspns of gelatin. Spread carefully over avocado layer. Cover dish tightly with plastic wrap and refrigerate overnight.
Just before serving, place caviar in fine sieve and rinse gently under cold running water. Sprinkle with lemon juice. Drain. Lift mold our of dish using foil extensions as "handles." Transfer mold to serving dish using wide spatula. Spread caviar over top. Cut a thin slice of lemon to the center and twist it to add to the top center of the caviar. Serve with thin slices of dark pumpernickel bread.