In a small bowl, mix the onion with sour cream and lemon juice. Place 1 blini on each plate, spread it with 1 tablespoon of the sweet-onion sour cream, and pile the trout on top. Top with caviar.
To serve, place two scoops of eggnog ice cream into 8 small bowls. Top with a teaspoon of caviar and scatter with the croutons. Yields one quart.