Stir together ingredients and slather on the lamb an hour ahead of grilling.
Get the grill hot to sear the outside.
Sear the non-fat side until nice black grill marks form.
Flip it fat side down and turn the heat down to low, close the lid.
Be really careful for flare-ups as the fat starts dripping.
Once the fat is a bit rendered and has browned, take it off direct heat and turn it back fat side up.
Keep the indirect heat high so the meat roasts fairly quickly- it should take about 20-30 minutes to reach medium rare- ~135°C in the thick part under the fat cap.
The outside should be an even dark brown with blackened areas.
Remove to a cutting board to rest.
To carve, start on the top (non-fat) side and run a knife along the middle bone and down along the ribs to the end to remove the smaller fillets.
Same process for the thicker side. Leave the fat on as it should be rendered completely and adds flavour.
Slice into medallions or leave whole to plate (medallions look nice but cool faster).