Dice beef into fairly large bite-sized pieces & season with S&P. Heat 1 tbsp oil in large deep heavy-based pot over high heat. Sear beef in 2 batches (don't cook right through).
Lower heat to medium-high & add remaining 2 tbsp oil. Fry carrot & onion until starting to soften/turn golden then fry garlic for 30 secs.
Lower heat to medium & stir in flour. Stir in wine to form smooth paste then gradually add in stock, stirring as you go to avoid lumps forming. Stir in the Worcestershire sauce, tomato paste, Oxo, bay leaves & beef (along with all the resting juices).
Bring to simmer then add lid, reduce to low and gently bubble for 1.5 hours, stirring occasionally. Remove lid & simmer 30mins or until beef is tender & sauce thickened to gravy-style consistency (see video). Will thicken more in next step.
Pour filling in baking dish (mine is 12x9" / 30x22cm) leave at least 30mins to cool until skin forms on top. Meanwhile melt butter & combine with rosemary, garlic, S&P. Very thinly slice potatoes (approx 3-4mm or ⅛") & pre-heat oven to 180C/350F.
Tightly stack 3 layers of potatoes with butter in-between & on top. Tightly cover in foil & bake 40mins. Remove foil, increase temp to 200C/400F & bake 20-25mins, or until potatoes knife tender. Can flick on grill to crisp potatoes.
