1. Discard head and skin of trout, if present.
2. Flake remaining trout fillets.
3. Puree cream cheese, sour cream, and horseradish in food processor.
4. Add fish and pulse to make a smooth dip.
5. Transfer to a bowl and stir in onion, parsley, and lemon juice.
6. Refrigerate, covered, at least one hour.