Personally, when I eat a lamb burger (or a nice rack), I want the full-on, musky, grassy flavour assault you get from larger, fattier lambs. If you want to be sure no one is —ahem—pulling the wool over your eyes, you can generally (but not always) tell by the size of the racks or legs.
NZ/AUS racks will have an eye of meat about as big as a 50¢ piece, Spain will be a little larger, whilst an American rack will be more like the size of, say, half an iPhone.
NZ/AUS legs will be small, like the rear leg of a medium-sized dog, Spain will be larger again, while an American lamb leg will be about the size of Arnold Schwarzenegger's shoulder and bicep if you know what I mean.
Price per Carton. Brand: Moralejo, Country: Spain
