Preheat the oven to 200°C (400°F) (180°C/350°F fan-forced).
Heat the olive oil in a large, deep, heavy-based oven-safe pan over medium heat.
Add the onion, celery, carrot, zucchini, garlic and salt. Cook, stirring, for 4–5 minutes, or until softened.
Add the thyme and beef and cook, stirring, for 3–4 minutes or until cooked through, breaking up the beef with a wooden spoon as you go.
Stir through the tomato paste until well combined.
Stir through the plain flour until dissolved.
Add the red wine vinegar, beef stock, Worcestershire sauce and pepper. Stir to combine. Simmer, uncovered, stirring occasionally, for 10 minutes or until thickened to a gravy consistency.
Remove the pan from the heat and smooth the top, so the filling cooks evenly. Allow the filling to cool slightly.
Place one-third of the potato slices in a layer across the beef, overlapping the slices slightly. Drizzle over some of the butter, then repeat with a second layer of potatoes, another drizzle of butter, then a final layer of potatoes and a final drizzle of butter. Finish with a sprinkling of salt.
Cover with foil or a lid and bake for 20 minutes. Uncover and continue to bake for a further 30 minutes, or until the potatoes are golden brown and are tender enough to be pierced with a fork.
Allow to sit for 10 minutes before serving, sprinkled with the extra thyme.
