Slowly cook onions in olive oil over low-medium heat until deeply caramelised; midway add brown sugar and red wine vinegar; cook to a jammy consistency. Cool slightly.
Lay out tenderised steaks; season lightly with salt and pepper. Place a tablespoon of onion mixture near one end; roll up and secure with cocktail sticks.
Grill over medium-high heat (or barbecue) turning until browned and cooked through (≈6–8 minutes) OR bake covered at 180°C / 350°F for 30–40 minutes with a splash of red wine.
Rest briefly; serve hot (also good cold next day).