Cottage Pie Potato Skins

Ingredients

• 500g beef mince
• 1 onion
• 1 carrot, finely diced
• 2 ribs celery, finely diced
• 1 tbsp flour
• 1 tbsp tomato paste
• 500ml beef stock
• 1 tbsp Worcestershire sauce
• 1 tsp garlic powder
• salt and pepper, to taste
• unsalted butter
• 150g frozen mixed vegetables
• 4 whole potatoes
• 100g mozzarella, grated
• 100ml milk
• 20g unsalted butter
• parmesan, grated
• chives, finely chopped