Remove the rack of lamb from the fridge at least 2 hours before cooking to bring it up to room temperature. Pat it dry and trim off most of the fat. You can leave a very thin layer. Season the lamb well with salt & pepper.
Preheat the oven to 190C / 375 F and heat an oven-proof cast iron skillet on low heat and add the splash of olive oil. Gently sear the meat on all sides over a low heat starting on the fat side to render it off. You don’t want to create too much of a crust.
Once seared add the butter to the pan along with the thyme and baste this over the lamb a few times. Cover the top with some of the buttery thyme leaves and baking in the oven for 4 minutes. Remove from the oven and remove the racks from the pan to allow to cool slightly.
Make the breadcrumb mix in a food processor. Add all the ingredients and whizz until you have a fine green crumb texture. Spread this out into a shallow dish.
Turn the oven temperature to 200C / 400F.