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 (see note 3 if you don’t have an oven-safe pan). 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. If you are using a baking dish, transfer the mixture to the dish now.
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 – the top will be crispy.
Allow to sit for 10 minutes before serving, sprinkled with the extra thyme.
Note 1 – The vinegar can be left out completely or substituted with apple cider vinegar. I do recommend leaving the vinegar in, though, as it adds a brightness to the gravy and cuts through the buttery potatoes and rich beef. Delish! Note 2 – It’s best to use a floury potato variety, like sebago (or russet for the US, or Maris Piper or King Edward for the UK) as they produce the fluffiest result without being too “slippery” when cooked. It’s easiest to slice the potatoes finely using a mandoline, or you can use a knife – aim for a thickness of 1–2 mm (¹/₁₆ inch). If the slices are thicker, you may need to increase the cooking time by an additional 15–20 minutes, or until the potatoes are tender and cooked through. Note 3 – If you do not have an oven-safe pan, you can use a regular pan, then later transfer the filling to a baking dish.
