Grilled Flank Steak with Mushrooms

Ingredients (serves 4): 1kg Flank or sirloin steak Salt Olive oil Black pepper 1kg Mixed mushrooms (if you can, make sure the assortment includes some shiitake mushrooms, they’re especially flavourful), cleaned, rough chop 2 tablespoons butter 1/2 cup minced shallots (or onions) 1 cup red wine (or beef broth) 1 Tbsp minced fresh rosemary   …