For the trout, heat a frying pan until hot and add half of the butter.
Season the trout with salt and freshly ground black pepper, to taste, and place in the pan.
Cook for two minutes, before carefully turning over with a fish slice and frying for a minute more. Remove the fish from the pan and set aside in a warm place.
Heat a separate frying pan and add the remaining butter. Add the rhubarb, sugar and ground ginger and stir fry briefly until the rhubarb has just softened and the sugar has dissolved. Set aside in a warm place.
For the croûtons, slice the crusts from the bread and cut into 1cm/½in cubes.
Heat a frying pan until hot and add the olive oil. Add the cubed bread and salt and freshly ground black pepper, to taste. Fry for 2-3 minutes until golden-brown and crunchy.
Transfer the fried bread to a plate lined with kitchen paper.
For the salad, place the lemon juice and zest into a small bowl and add the mustard. Season with salt and freshly ground black pepper and whisk in the olive oil.
Place the mizuna, baby spinach and flatleaf parsley leaves into a bowl, along with the croutons and the dressing and mix to combine.
Divide the trout equally among four plates and top with some of the rhubarb. Place a pile of salad alongside and serve.