Place potatoes in a pot, cover with cold water, and season with salt. Boil 15–18 minutes until fork-tender. Drain well. Mash or rice until smooth. Stir in the butter, warm milk, and egg yolk; season with salt and pepper. Set aside.
Cook the Beef Filling
Heat olive oil in a large sauté pan over medium heat. Add onion, carrot, and celery. Cook 8–10 minutes until softened and lightly golden. Add garlic and cook 1 minute. Increase to medium-high, add beef mince, break apart, and brown (7–8 minutes). Season lightly.
Stir in the tomato paste and cook for 2 minutes to caramelise. Add Worcestershire, balsamic, beef stock, thyme, rosemary and bay leaf. Scrape the pan well. Simmer gently 25–30 minutes until rich and slightly thickened. Remove herbs/bay leaf. Stir in peas. Season to taste.
Assemble & Bake
Divide the beef filling evenly into 5 × 750 ml oven-safe containers. Spoon the mash on top and spread it evenly to the edges. Rough up the surface with a fork. Place the containers on a tray and bake at 200 °C (390 °F) for 20–25 minutes, or until golden.
Chef's Tips
Grate the onion, carrot and celery rather than dicing them — grated veg melts down into the sauce as it simmers rather than staying chunky, giving the filling that classic smooth, rich texture cottage pie is known for.Let the tomato paste caramelise the full 2 minutes before the stock goes in — skipping this step is the easiest way to end up with a filling that tastes flat instead of deep and savoury.Rough up the mash surface with a fork before baking, as written — those ridges are what catch and crisp up in the oven, giving you the contrast between soft mash and golden, slightly crunchy peaks.
Storage & Reheating
Refrigerate for up to 4 days, or freeze for up to 3 months, in a sealed container — cool completely before sealing.To reheat, oven is best: 180°C (355°F) for 20 minutes from the fridge, or 25 minutes from frozen. If using the microwave, cover loosely and heat for 4–5 minutes, stirring halfway for a softer crust. Thaw frozen portions in the fridge overnight before reheating for the best texture.