Put the chicken into a wide ceramic bowl. Pour 2 tablespoons of the vinegar into a bowl and stir in 3 tablespoons of the soy sauce, the red chilies, the peppercorns and half the sugar substitute. Pour this mixture over the chicken and toss to mix well. Cover the bowl and marinate in the fridge for about 30 minutes.
In a small bowl, combine the remaining vinegar, soy sauce and sugar substitute. Stir in the tomato paste and stock and whisk until smooth.
Spray a heavy-based frying pan with low calorie cooking spray and place over a high heat. Add the chicken mixture and stir-fry for 5-6 minutes or until browned and just cooked through.
Add the scallions, garlic, red bell pepper, bamboo shoots and water chestnuts and stir-fry for 3-4 minutes.
Add the stock mixture and bring to the boil. Reduce the heat to low and cook gently for 5-6 minutes, stirring continuously, until the sauce has thickened. Check the seasoning.
Line a shallow serving dish with the baby spinach leaves. Spoon over the chicken mixture and serve immediately.