For the salad, mix together the spinach, watercress and coriander cress in a bowl. Add half of the diced mango and mix again.
Place the remaining diced mango and the white wine vinegar into a food processor and blend to a purée.
With the motor still running, slowly add the oil in a thin stream until the mixture is just emulsified. Season, to taste, with salt, freshly ground black pepper and a pinch of sugar.
Spoon the mixture over the salad and mix to combine. Set aside.
For the salmon, preheat the grill to its highest setting.
Place a 20cm/8in chefs’ ring (or bottomless tart ring) onto a baking tray. Layer one quarter of the salmon slices into the ring, overlapping each slice a little.
Mix the melted butter and vanilla seeds together in a bowl and season, to taste, with salt and freshly ground black pepper.
Brush the melted butter mixture over the salmon layer, then remove the chefs’ ring. Repeat the process with the remaining salmon and butter mixture to create four rings of salmon.
Place the salmon rings under the grill for 2-3 minutes, or until just cooked through. Slide each one onto a serving plate.
Meanwhile, for the tempura prawns, heat the oil in a deep-fat fryer to 190C/375/Gas 5. Alternatively, fill a deep-sided frying pan two-thirds full with vegetable oil. Heat the oil until hot but not smoking. (Caution: hot oil can be dangerous. Do not leave unattended.)
Place the flour, cornflour and salt into a bowl and add the sparkling water, whisking well until the mixture forms a thick, smooth batter.
Drop the prawns into the batter to coat fully, then place straight into the hot oil. Fry for 2-3 minutes, or until the batter is golden-brown and crisp and the prawns are pink and cooked through. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper.
To serve, pile a portion of salad onto each circle of salmon. Top with three tempura prawns.