For the orange hollandaise, heat the lemon juice and white wine vinegar in a non-reactive pan until just boiling.
Meanwhile, heat the butter in a heavy-based pan over a low to medium heat until melted. (NB: Do not overcook the butter or it will burn.)
Blend the egg yolks and salt in a food processor until well combined.
With the motor running, gradually add the hot lemon and vinegar mixture in a thin stream, until all of the mixture has been fully incorporated into the egg yolks.
With the motor running, gradually add the melted butter in a thin stream, until all of the butter has been fully incorporated into the mixture and the sauce has thickened.
Transfer the hollandaise sauce to a bowl, then stir in the orange zest and season, to taste, with salt and freshly ground black pepper. Set aside (keep warm).
For the trout and orange salad, heat 25g/1oz of the butter in a pan over a medium heat. When the butter is foaming, add the trout fillets and fry for 2-3 minutes on each side, or until cooked through. (NB: The trout is cooked through when the flesh is opaque.) Remove from the pan and keep warm.
Heat the remaining 25g/1oz of butter in a separate frying pan over a medium heat. When the butter is foaming, add the orange and grapefruit segments and fry on both sides until caramelised.
Meanwhile, blanch the sliced fennel and shallots in a large pan of boiling, salted water. Drain well, then refresh in ice-cold water. Drain again.
In a large bowl, mix together the blanched fennel and shallots, caramelised orange and grapefruit and chopped parsley until well combined.
Whisk together the extra virgin olive oil and the juices reserved from the segmented orange and grapefruit. Season, to taste, with salt and freshly ground black pepper.
To serve, divide the fennel and grapefruit salad equally between two serving plates. Place one trout fillet on top of each portion. Drizzle over the orange hollandaise. Drizzle the oil and citrus dressing around the edge of the plates.