Line a strainer with cheesecloth or a tea towel and place it over a large bowl. Place the yoghurt on the cheesecloth and refrigerate overnight. The whey from the yoghurt will drip into the bowl and you'll be left with very thick yoghurt solids. Roll these solids into balls about the diameter of a 10-cent coin, transfer to a small container and cover with olive oil. Refrigerate until ready to use. (The labne will keep in the fridge for about a week.)

Heat the olive oil in a large frying pan (reserving a little of the oil to cook the lamb) over low heat and add the garlic. Cook for about 10 minutes, stirring regularly until well browned. Increase the heat to high, add the mushroom slices and spring onions, season with salt and pepper and fry for about 8 minutes until the mushroom has browned. Transfer the mushroom mixture to a bowl.

Return the frying pan to high heat without washing it. Add a little more olive oil, season the lamb well with salt and pepper and fry for about 3 minutes. Flip the lamb and add the butter and rosemary to the pan. Spoon the melted butter over the lamb for a further 3-5 minutes until the lamb is cooked to your liking. Transfer the lamb and any remaining butter and rosemary to a warm plate to rest. 

Toast the sourdough on a griddle pan and place on serving plates. Slice the lamb and place over the toast with any butter and resting juices. Top with the mushroom mixture and a few balls of labne. Season with a little more salt and pepper and serve.

Adam's tip Making labne doesn't have to be a slow process. If you fold the cheesecloth double or triple and gently squeeze the yoghurt to extract the whey, you can make it immediately. It takes a bit more effort but it's a good work around if you're short on time.