Hearty Bangers and Mash Pie

Ingredients

• 750 g store-bought mashed potatoes
• 8 pork sausages, each sliced into 4 or 5 pieces
• low calorie cooking spray
• salt and black pepper
• 1 medium egg
• 1 celery stick, chopped
• 300 ml beef stock
• 2 tbsp balsamic vinegar
• 1 tbsp Worcestershire sauce
• 1 tsp garlic powder
• 1/2 tsp mustard powder
• 3 tbsp tomato puree
• 1/2 tsp dried thyme
• 2 onions, chopped
• 200 g frozen peas