Preheat the oven to 200°C (390°F) or 180°C (360°F) for fan-forced ovens. Grind the peppercorns in a small food processor (or spice grinder, mortar and pestle) until it resembles cracked pepper.
Heat oil in a large deep frying pan over medium-high heat, then add the garlic and onions. Stir regularly for 1 minute or just when the onion slightly softens. Add in the mushrooms and stir occasionally for 2-3 minutes. Place in the mince and break up as you stir, then cook the beef until it turns brown.
Once the beef has browned, mix in the tomato paste and cook for two minutes, then mix in the flour until combined. Pour in the beef stock, then add the stock cubes, Worcestershire sauce, horseradish, peppercorns and salt. Bring the mixture to a boil, then reduce the heat to medium-low and cook uncovered for 10 to 15 minutes or until the sauce has reduced and thickened. Have a taste and add more salt or pepper if needed. Transfer the mixture to a heat-proof bowl and leave to cool for at least 30 minutes.
Divide the mixture between 8 x ramekins (1 cup capacity). Cut out 8 x 12cm/4.7in circles from the pastry sheets (4 circles on a sheet, I used a bowl to trace my circles). Place the pastries over the top of the ramekins and press the sides down to seal them, then lightly brush the pastry with egg. Use a fork to poke holes in the middle of the pie to allow steam to escape. Bake the pies for 30 minutes or until the pastry is golden and flaky.
Notes
(NOTE 1) Whole black peppercorns - For that delicious vibrant and 'peppery' taste, it's worth paying extra for whole peppercorns. Pre-cracked or ground pepper won't give the same beautiful spicy flavours as you would get with freshly ground peppercorn. To grind your peppercorns, use either a mortar and pestle, spice grinder or a powerful mini food processor.(NOTE 2) Mushrooms - I used small button mushrooms which have a mild flavour, but Swiss Brown and white mushrooms are good too. If you can't find small mushrooms, just cut them into quarters or more depending on their size. Mushrooms soak up water like a sponge, so it's best not to wash them in water. Clean them by using a damp tea or paper towel and gently wipe the mushroom caps clean.(NOTE 3) Beef mince (ground beef) - I used regular mince with some fat mixed in as it tastes better, however, lean mince would also work.(NOTE 4) Beef stock or broth - I prefer using salt-reduced beef stock because I have more control over the sodium levels in my cooking. It's always better to be under-seasoned than over-seasoned as it's easier to fix.(NOTE 5) Frozen pre-rolled puff pastry - For convenience, I used prepackaged ready-rolled pastry. I used 24cm (9.5in) square pastry sheets. Follow the instructions on the packet on how to defrost the pastry sheets, but don't leave them out for too long. They'll go elastic and break easily, making them harder to work with.A Third-Party Application calculated the calories and nutritional information. Please use this as an approximate guide only.Cooking measurements are in Australian standard spoon and cup measurements.