Mushroom Stuffed Flank Steak Roll

1 beef flank steak (about 800 g)
½ cup lemon juice
½ cup soy sauce
½ cup honey
2 tsp ground mustard
1 tsp SIDS SALT & PEPPER
½ tsp SIDS CRAZY LEMON
MUSHROOM FILLING:--
500 g sliced fresh mushrooms
1 medium onion, finely chopped
¼ cup butter, cubed
½ cup minced fresh parsley
2 tbsp cornflour
2 cups beef stock

Cut steak horizontally from a long side to within 1 cm of opposite side. Open steak so it lies flat then cover with plastic wrap. Flatten to 5 mm thickness.
In a small bowl, combine the lemon juice, soy sauce, honey, mustard, SIDS SALT & PEPPER and SIDS CRAZY LEMON. Pour half of marinade into a large shallow dish, add beef and turn to coat then cover and refrigerate overnight. Cover and refrigerate remaining marinade.
In a large frypan, sauté mushrooms and onion in butter until tender. Remove from the heat and stir in parsley. Drain and discard marinade. Open steak, spoon filling over steak to within 1 cm of edges. Roll up tightly jelly-roll style, starting with a long side. Tie with kitchen string.
Place in a greased shallow roasting pan. Pour reserved marinade over steak.
Cover and bake at 175°C for 45 minutes. Uncover and baste with pan drippings. Bake 30 minutes longer until meat reaches desired doneness (for Medium-rare, a thermometer should read 57°C. Medium, 60°C. Medium-well, 63°C) Remove and keep warm. Meanwhile, transfer cooking juices to a saucepan and skim fat.
Bring cooking juices to a boil. Mix cornflour and stock until smooth. Stir into cooking juices. Bring to a boil then cook and stir 1-2 minutes until thickened. Cut steak into slices and serve with sauce.