Step 1 Set a large non-stick frying pan over medium heat. Heat olive oil. Add onion, garlic, carrot, celery and mushrooms; cook, stirring often, for 5 minutes, or until soft. Increase heat to medium high.
Step 2 Add beef mince to frying pan and stir with a wooden spoon (to break up) for 4–5 minutes, or until browned. Scatter mince with flour and cook, stirring, for 1 minute. Stir pasta sauce, tomatoes and Worcestershire sauce into mince; bring to the boil. Reduce heat to medium low. Cover pan partially and simmer, stirring occasionally, for 15 minutes; stir in frozen peas.
Step 3 Meanwhile, boil potatoes in a large saucepan of boiling water for 10–12 minutes, or until tender. Drain potatoes and return to pan. Reduce heat to low. Toss potatoes until dry, then remove from heat; add milk and mash until smooth.
Step 4 Preheat oven to 200°C. Line a large baking tray with a sheet of baking paper. Grease 4 x 2-cup ovenproof, microwave-safe dishes. Spoon mince mixture into dishes and top with mashed potato. Put dishes on prepared tray and bake for 15–20 minutes, or until edges bubble and potato is just golden. Leave pies to cool slightly. Cover dishes with cling film and freeze.
To reheat: Transfer pies to fridge to thaw for a day (or overnight). Remove cling film; discard. Cover pies and microwave (individually) on 70% power for 6–8 minutes, or until edge is bubbling, and serve.