For the fennel and saffron broth, bring the stock, saffron and pastis to the boil.
Meanwhile, heat half of the butter in a frying pan and fry the fennel and shallots over a low heat for 2-3 minutes, or until softened.
Pour in the white wine, bring to the boil and continue to cook until the volume of the liquid has reduced by half.
Add the stock mixture to the pan and bring to the boil. Reduce the heat until the mixture is simmering and simmer for 1-2 minutes. Whisk in the remaining butter and season, to taste, with salt and freshly ground black pepper. Stir in the chives and set aside.
For the fennel frond tempura, heat the oil in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Whisk the cornflour and sparkling water together in a bowl to form a smooth batter. Dip the fennel fronds into the batter, shaking off any excess, then fry in the vegetable oil for 20-30 seconds until pale golden-brown and crisp. (Do this is batches if necessary.)
Remove from the oil with a slotted spoon and set aside to drain on kitchen paper. Season, to taste, with salt.
For the trout darnes, season the trout, to taste, with salt and freshly ground black pepper.
Heat the butter and oil in a frying pan and fry the trout darnes for 2-3 minutes on each side, or until cooked through.
Remove the trout darnes from the pan and set aside to rest. Squeeze over lemon juice, to taste.
To serve, reheat the fennel and saffron broth, if necessary, and ladle into four shallow soup bowls. Top each with a trout darne and some fennel frond tempura.
