Steak & Kidney Pie With English Mustard Gravy

1.5 kg beef, cubed
olive oil
1 large onion, diced
2 celery stalks, diced
2 medium carrots, diced
1 garlic clove, crushed
3 tbsp plain flour
¾ litre beef stock
2 cans diced tomatoes, drained a little
1-2 heaped tbsp English mustard
freshly ground salt & pepper
5 lamb?s kidneys, halved, cored & chopped
3 tbsp freshly chopped parsley
2 puff pastry sheets
1 egg
4 tbsp milk
Seal beef in hot oil and drain well. Then remove any excess oil from the pot and gently saute onion, celery, carrots and garlic until softish. Add flour and briefly cook, continually stirring. Add stock, tomatoes and mustard. Mix well, season and return beef to the pot, together with kidneys and parsley. Stir well and bring to the boil. Turn heat down, cover and gently simmer for about 1½ hours until tender. Then remove from heat and allow to cool a little. When ready, preheat oven to 190°-200°C. Transfer mixture to a gratin (pie) dish and top with a pastry sheet, cutting off any edges. Cut leaf shapes out of remaining pastry sheet. Cut soft lines (not all the way through) on top and arrange on top of pie. Mix together egg and milk. Then generously brush pastry with egg wash and cook in oven for about 30 mins until risen and golden brown.