Preheat oven to 160˚C / 140˚C fan-forced. Place flour into a large snap-lock bag. Season. Add lamb shanks and toss to coat in flour. Shake off excess. Heat 1 tbs oil in a large ovenproof pan over medium-high heat. Brown lamb shanks until golden. Set aside.
Add remaining 1 tbs oil to the pan. Add onion, garlic, celery, carrots, mushrooms and rosemary. Cook, stirring often, for 3-4 minutes until onion softens.
Stir in tomato paste. Cook for 1 minute. Stir in stock and tomatoes. Cover and bring to the boil. Return lamb shanks to pan. Cover. Bake for 2 hour - 2 1/2 hours until lamb is very tender and sauce has thickened. Set aside to cool slightly.
Remove lamb shanks to a plate. Remove meat from bones, chop and add to mushroom mixture. Spoon into 4 x 1½ cup capacity greased oven-proof dishes (top 10cm wide).
Increase oven to 190°C fan-forced. Place pastry onto a board to defrost. Cut 4 x 11cm rounds from pastry. Top pies with pastry. Press edges with a fork to seal. Brush with egg and cut a small cross into the centre of each. Place pies onto a baking tray lined with baking paper. Bake for 25-30 minutes or until pastry is puffed and golden. Serve.