Combine oils in bowl. In separate bowl, combine mustard, shallots, onion, egg yolk, hard - boiled egg, salt, pepper and vinegar. Blend briskly with wire whisk. Slowly dribble oils into bowl to create emulsion. Add horseradish, tarragon and parsley. Blend well with whisk. Serve sauce around trout. Garnish each serving with lemon wedge.