For the marinated steak:
1) Place the steak pieces between two pieces of cling film or parchment and pound to 1/2-cm thickness. Cut into four equal pieces, diagonally, against the grain.
2) In a small bowl, whisk together 15ml of the olive oil, balsamic vinegar, sugar and garlic. Put the steak into a sealable plastic bag with the marinade and let sit for 30 minutes at room temperature, or one hour in the refrigerator.
3) Spray a large griddle pan with cooking spray and preheat over medium-high heat. Cut the onion into 0.5cm rounds, brush both sides with the remaining tablespoon of oil and griddle for 6 minutes per side. Separate into rings and set aside.
4) Remove the meat from the marinade and season both sides with salt and pepper. Discard the marinade. Cook the meat in the griddle pan over medium-high heat for about 3 minutes per side for medium rare.
5) Arrange three radicchio leaves and a pile of onions on each serving plate. Top with a piece of steak. Drizzle with blue cheese sauce and top with a sprinkling of parsley.
For the blue cheese sauce:
1) In a small bowl combine the blue cheese and buttermilk with a fork, mashing until creamy. Stir in the Worcestershire and set aside.
2) Serve with the steak.