Dice the beef into fairly large bite-sized pieces and season with salt and pepper.
Heat 1 tbsp oil in a large deep heavy-based pot over high heat. Once hot, spread out half of the beef and fry until brown all over (don't try and cook it right through). Remove and repeat with the second batch. Don't overcrowd the pot or the beef will steam and stew. Add more oil if needed.
Lower heat to medium-high and add the remaining 2 tbsp oil. Fry the carrot and onion until they soften and begin to very lightly brown, then fry the garlic for a further 30 seconds or so.
Lower the heat to medium and stir in the flour. Pour in the wine and give it a really good stir until it thickens to a smooth paste. Gradually add in the stock, stirring as you go to avoid lumps forming. Stir in the Worcestershire sauce, tomato paste, Oxo cube, bay leaves and cooked beef (along with all of the resting juices).
Bring to a simmer, then pop on the lid and reduce the heat to low. Allow to gentle bubble away for 1 hour and 30 mins, stirring occasionally. Remove the lid and simmer for a further 30 mins, or until the beef is tender and the sauce has thickened to a gravy-style consistency (see video). Keep in mind it'll thicken more in the next step. Adjust seasoning if desired.
Pour the filling into a large baking dish and allow to cool for at least 30 mins, until a skin forms across the top. Meanwhile, melt the butter and combine in a small mixing bowl with rosemary, garlic, salt and pepper. Very thinly slice the potatoes (approx 3-4mm or 1/8") and pre-heat the oven to 180C/350F.
Layer 1/3 of the potatoes across the top. Brush with 1/3 of the butter, then repeat two more times, ensuring you layer the potatoes in a way that reduces as many gaps as possible.
Tightly cover in foil then bake in the oven for 40mins. Remove the foil, increase the oven temp to 200C/400F and bake for a further 20-25mins, or until the potatoes are knife tender. You can flick on the grill to brown them further if you'd like.
Allow to cool for a few mins, then serve up and enjoy!