For the salmon, using a pestle and mortar pound the garlic and ginger with the salt to form a smooth paste. Spoon into a shallow dish and mix in the lime juice.
Chop the salmon into 2.5cm/1in cubes and add to the dish. Mix gently to coat evenly in the mixture. Leave to marinate for about 10 minutes.
For the coriander marinade, place the spinach, coriander and mint into a food processor. Add the chillies, a squeeze of lime juice and a trickle of vegetable oil. Blend until smooth, adding a little more oil if the mixture is too thick. Spoon into a bowl and mix in the flour and yoghurt. Season with a little salt, then add the marinated salmon, coating evenly in the mixture.
Preheat the oven to 200C/390F/gas 6. Grease 1-2 large baking sheets with olive oil.
Place the marinate salmon pieces onto the prepared trays, spacing them apart. Roast for 8-10 minutes, or until just cooked through. Remove from the oven and set aside. Preheat the grill to high.
For the garnish, mix the sugar, reserved pickling liquid from the beetroot, lime juice and coriander leaves together in a bowl. Add the shallot rings and mix well to coat.
Place the trays of salmon under the grill and cook for 3-4 minutes, or until golden-brown.
To serve, spoon the grilled salmon into small serving dishes and garnish with the shallot mixture and chopped beetroot.
