Add onion to butter and place in microwave until butter is bubbly and onions are softened. When butter has cooled stir in eggs. Line a small shallow dish with plastic wrap and pour butter mixture onto plate. Refrigerate until butter has hardened. Mix sour cream with salmon and lemon juice. Turn butter mixture out of mold and cover with sour cream. Top with a layer of caviar, either red or black or a decorative combination of the two. Serve with crackers. This can be made early in the day, adding caviar just before serving. Must be kept cold until served.