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.

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.

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.