Cut the chicken into 2.5 cm (1 inch) cubes and place in a wide glass or ceramic bowl.
Mix together 2 tablespoons of the vinegar, 3 tablespoons of the soy sauce, 1 tablespoon of the cornflour, the chillies, Szechuan peppercorns and half the sugar in a small bowl. Pour over the chicken and toss to coat evenly. Cover and leave to marinate in the refrigerator for about 30 minutes.
Combine the remaining vinegar and soy sauce, tomato purée and stock with the remaining cornflour and sugar.
Spray a heavy-based nonstick frying pan with cooking spray and heat over a high heat until just smoking. Add the chicken and marinade and stir-fry for 5–6 minutes, or until browned and just cooked through. Add the spring onions, garlic, red pepper, bamboo shoots and water chestnuts and stir-fry for 3–4 minutes.
Add the stock mixture and bring to the boil, stirring constantly. Reduce the heat and cook gently, stirring, for about 5–6 minutes until the sauce has thickened. Check the seasoning and adjust if necessary.
Line a shallow serving dish with the baby spinach leaves. Spoon over the chicken mixture and serve immediately.