In a large, oven-safe pan, heat the olive oil over medium heat and sauté the onion, celery, carrot, zucchini, garlic, and salt for 4–5 minutes until softened.
Add thyme and minced beef, cook for 3–4 minutes breaking up the beef until cooked through.
Stir in the tomato paste and flour until dissolved, then add red wine vinegar, beef stock, Worcestershire sauce, and pepper. Simmer for 10 minutes until thickened.
Remove from heat and smooth the top. Allow to cool slightly.
Layer one-third of the potato slices over the beef, drizzle with butter, and repeat for two more layers, finishing with potatoes topped by a sprinkle of salt.
Cover with foil and bake for 20 minutes, then uncover and bake for an additional 30 minutes until golden brown.
Let it sit for 10 minutes before serving, garnished with extra thyme.
Notes
Grating zucchini can help with fussy eaters by hiding it in the filling.