Take some cooking twine or string and tie the Barnsley chops around the edge to help them hold their perfect round shape. This step is optional and they will cook fine without it.
Rub the chops with the cooking oil and season them with the salt, paying particular attention to the rind.
Heat a 28cm or 11" frying pan (not non-stick) over a medium to medium-high heat and sear the skin/rind of the chops all over until they begin to brown and the fat begins to render out of them. This will take approximately 7-10 minutes.
Pour the port and lamb stock into a small (15cm 6") saucepan and simmer over a medium-high heat until it has reduced to approximately 150ml (just over half a cup). You can do this whilst you are searing the lamb.
Bash the garlic cloves with the side of a knife.
Flip the chops onto one side and cook for 8-10 minutes; it should be a nice dark colour before flipping.
Flip over the chops, then add the butter to the pan; it should begin to foam. If it does not, increase the heat.
Add the garlic and rosemary, and then whilst the lamb cooks for another 3-4 minutes, baste the top of the lamb with the foaming butter.
Remove the lamb and place it on a board, cover with foil and rest for 5 minutes.
Return the pan to the heat and then sprinkle in the flour and cook, stirring constantly for 1-2 minutes.
Pour in the reduced port mix and cook, stirring constantly for a final 2-3 minutes.
Strain the port lamb sauce before cutting the twine from the chops and serving it with the lamb.
Notes
This recipe aims for medium rare and will cook a 30mm chop to 52°-54°C or 125°-129°F before resting. See the frequently asked questions section for cooking to a different stage of "doneness".