beef heart
The heart muscle of the cow, very lean, firm and fine-grained, with a robust, faintly metallic beef flavour.
Rarely found in supermarkets, but butchers will usually order it in, and Turkish or halal butchers often have it in stock. Lean beef from the leg or lamb heart can stand in.
Substitute:
Lean beef from the leg comes close in firmness and leanness but lacks the faintly metallic offal note. Lamb heart tastes similar and is more tender, but much smaller.