Line a baking tray with aluminium foil and grease lightly with oil. Place the trout onto the prepared tray and cover with a second sheet of aluminium foil. Bake in the oven for 12-15 minutes, or until the fish is cooked through. (NB: The fish is cooked through when the flesh nearest the bone at the thickest part is opaque.)
When the trout is cooked through, set aside to cool. When it is cool enough to handle, remove the skin and discard. Flake the fish into a sieve suspended over a bowl, discarding any bones.
Meanwhile, melt the butter in a large pan over a medium heat. When the butter is foaming, add the onion and fry for 2-3 minutes, or until starting to colour.
Stir in the flour and continue to heat for 30 seconds, stirring continuously.
Add the milk and continue to cook, stirring continuously, for 4-5 minutes, or until the mixture has thickened.
Remove the pan from the heat and beat in the tomato purèe, tarragon, Dijon mustard and cream until well combined. Season, to taste, with salt and freshly ground black pepper. Cover the pan with cling film and set aside to cool slightly.
When the mixture is tepid, stir in the trout flakes until well combined. Cover again and set aside until completely cooled, then chill in the fridge for a minimum of five hours, or until solid.
When the trout mixture has cooled, make quenelles or balls of the mixture using two large spoons to form croquettes.
Pour the beaten eggs into a shallow bowl. Sprinkle half of the breadcrumbs and half of the ground almonds into another shallow bowl. Sprinkle the three tablespoons of flour into a third bowl.
Dredge each croquette lightly in the flour, then dip it into the beaten egg, then roll in the breadcrumb mixture until completely coated. Place the prepared croquette onto a baking tray. Repeat the process until you have prepared eight croquettes.
When eight of the croquettes have been prepared, discard the breadcrumb mixture and replace it with the remaining breadcrumbs and almonds. Repeat the process with the remaining croquette mixture until you have 12-16 croquettes.
Chill the croquettes in the fridge for at least 30 minutes.
Half-fill a large pan with sunflower oil and heat to 180C (use a cooking thermometer to check the temperature).
When the oil is hot, carefully lower the croquettes, in batches, into the pan using a slotted spoon. Fry for 2-3 minutes, turning once during cooking, or until the breadcrumbs are crisp and golden-brown. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper. Sprinkle with salt and keep warm.
Repeat the process with the remaining batches of croquettes.
To serve, pile the croquettes into the centre of a serving plate and squeeze over the juice from the lemon wedges.