Toss steak in 1 tablespoon of the oil. Heat a large non-stick frying pan over a medium-high heat. Cook meat, in 2 batches, for 5 minutes or until browned all over. Set aside. Reserve pan.
Heat ¼ cup of the oil in a stockpot over a medium heat. Add onion, carrot and garlic. Cook, stirring, for 4 minutes. Add all mushrooms and cook, stirring, for 5 minutes. Meanwhile, combine flour, gravy powder and ½ cup of the stock in a jug. Set aside. Pour remaining stock into reserved pan with tomato paste and Worcestershire sauce. Put over a medium-high heat. Add gravy mixture, then cook, stirring, for 2 minutes or until combined and starting to thicken.
Return meat to pot with vegetables. Add gravy mixture. Season and stir to combine. Cover and simmer, stirring occasionally, for 45 minutes. Uncover and keep at a gentle simmer for a further 20 minutes, stirring occasionally, or until meat is tender and sauce is thick. Remove from heat.
Meanwhile, preheat oven to 180°C. Thinly slice potatoes, leaving skin on. Combine with remaining oil and thyme. Season with salt. Spread in a single layer on 3 large oven trays. Bake for 15-25 minutes or until golden.
Increase oven to 200°C. Divide beef mixture between 6 x 1½-cup capacity ovenproof dishes. Cut puff pastry to fit tops of pies. Trim, press edges to seal, brush with egg and cut small slits in centres. Transfer to an oven tray. Bake for 20-25 minutes. Garnish with thyme and serve with potatoes.