In a medium bowl, place eggs, mayonnaise, celery, cornichons, mustard powder, apple, capers, salt and pepper, to taste. Gently mix until combined.
Spread 1 side of each slice of bread with mayonnaise. Line half of the slices with radicchio or arugula leaves. Spread with an even layer of egg salad. Top with remaining bread. Cut in half and serve.
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