Bangers and Mash Pie - Pinch Of Nom

Ingredients

• 750 g potatoes, (peeled and quartered)
• 1 medium egg, (beaten)
• ½ tsp English mustard powder
• salt and black pepper
• 8 chicken chipolata sausages ((272g in total), each sliced into 4 or 5 pieces)
• 2 onions, (thinly sliced)
• 1 celery stick, (finely chopped)
• low calorie cooking spray
• 250-350 ml beef stock ((1 very low salt beef stock cube dissolved in 250-350ml boiling water))
• 2 tbsp balsamic vinegar
• 1 tbsp Henderson's relish
• 1 tsp garlic granules
• ½ tsp dried thyme
• 3 tbsp tomato puree
• 200 g frozen peas