Place butter in a small bowl. Use a fork to mash. Add garlic and rosemary and stir until combined. Place on a small piece of baking paper and shape into a log about 12cm long. Wrap log in baking paper. Place in the fridge to chill until firm. Cut into 12 slices.
Preheat the air fryer for 5 minutes at 200C. Spray lamb chops lightly with oil. Season. Arrange over base of air fryer basket in a single layer. Cook for 3 minutes each side for medium-rare. Top each chop with half a slice of garlic butter. Close the air fryer for 1 minute until butter has melted (do not turn the air fryer on).
