Bring the stock, saffron, and pastis to the boil in a saucepan.
Fry the fennel and shallots in half of the butter over low heat for 2-3 minutes until softened.
Add white wine and boil until the liquid reduces by half.
Add the stock mixture, simmer for 1-2 minutes, then whisk in remaining butter, seasoning, and chives.
Heat vegetable oil in a deep frying pan for the tempura.
Whisk cornflour and sparkling water into a smooth batter and dip fennel fronds in to coat.
Fry fronds for 20-30 seconds until pale golden, then drain on kitchen paper and salt.
Season the trout darnes with salt and black pepper.
Fry trout in butter and oil for 2-3 minutes on each side until cooked.
Rest the trout and squeeze over fresh lemon juice.
Ladle broth into bowls and top with one trout darne and tempura fronds.