Make the best shepherd's pie

Ingredients

• 1kg of potatoes
• 125ml of milk
• 200ml of beef stock
• 9 tablespoons of butter
• Black pepper and salt on demand
• 1 onion
• 5 cloves of garlic
• 2 carrots
• 4 sticks of celery
• 200mm fresh rosemary sprigs
• 1 cup of chopped parsley
• 400g of minced lamb
• 400g of minced beef
• 1 sachet of tomato concentrate
• 1 tablespoon of flour
• 1 cup grated Cheddar
• 1 or 2 tablespoons of coarse Dijon mustard
• 3 dashes of Worcester sauce