Put the salmon steaks on an oiled roasting rack or wire rack inside a roasting tin and pour 450 ml (¾ pint) boiling water into the tin. Cover tightly with foil and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes, or until the salmon is almost cooked through.
Meanwhile, put the tamarind in a small saucepan and blend in 175 ml (6 fl oz) water. Stir in the soy sauce, ginger, sugar, garlic and chilli and heat through gently for 5 minutes. Blend the cornflour with 1 tablespoon water and add to the pan. Heat gently, stirring, for 1–2 minutes, or until thickened.
Quarter the pak choi lengthways into wedges and arrange the pieces around the salmon on the rack with the spring onions. Re-cover and return to the oven for a further 8–10 minutes, or until the vegetables have wilted.
Stir the coriander into the sauce. Transfer the fish and greens to warm serving plates, pour over the sauce and serve.