Trim the fat on the piece of beef to about 2 cm/1 in thick.
Using a sharp knife, cut a crisscross pattern at 1 cm/1/2 in intervals across the fat.
Season lightly with salt, but add plenty of pepper.
Heat oil in a large heavy-based flameproof pot.
Add the beef and brown on all sides, leaving the fatty side until last.
Reduce the heat and slowly brown the fat; at the same time you will be rendering it (reducing the fat content).
Remove the beef from the pot and drain off the fat.
Melt the butter in the pot until foaming, then add the beef.
Add the bay leaves, garlic and 5 tablespoons of the vinegar.
Moisten a piece of baking paper large enough to cover the pot.
Push the damp paper down into the pot to rest on and cover the beef entirely.
Simmer over a low heat until the vinegar reduces by half.
Add a little more vinegar and when that reduces, add some more. Continue in this way until the beef is cooked to medium, 50–70 minutes.
The sauce should never completely reduce because you are trying to create an emulsion (a creamy mixture of the beef fat and the vinegar). Add water, if necessary.
Turn off the heat and rest for 10 minutes.
Slice the beef and serve.
Spoon a little of the sauce over the beef and garnish with the bay leaves and garlic.
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