Spray a wok or frying pan with cooking spray and heat over a medium heat. For the sauce, add the garlic, ginger, chillies and spring onions and stir-fry for about 2–3 minutes until softened. Stir in the crushed peppercorns, tomato purée and hot chilli sauce and cook, stirring, for a few seconds. Add the soy sauce, vinegar and stock to the pan, bring to a simmer and cook for about 2–3 minutes until thickened slightly.
Make 3–4 deep diagonal slashes in the skin of the fish with a sharp knife. Arrange the fillets on a baking sheet lined with baking paper, season with salt and brush all over with the sauce. Place in a preheated oven, 200°C (400°F), Gas Mark 6, for about 15–20 minutes until the fish is cooked through.
Meanwhile, spray a separate nonstick wok or frying pan with cooking spray and heat over a medium heat. Add the spring onions and stir-fry for about 1–2 minutes until softened. Add the red peppers, pak choi and measurement water and stir-fry for 3–4 minutes until the vegetables are just tender, then stir in the soy sauce. Spoon the vegetables into the centre of 4 warmed plates. Top each with a fish fillet, then spoon over some of the sauce. Serve with Quick Stir-fry Rice (see below).