Arrange onion halves snuggly in a small lined baking tray. Pour over combined sugar, vinegar and oil then season with salt and pepper. Roast on the top shelf of the oven for 20 minutes at 180°C.
Tie beef fillet with cooking string at 5cm intervals to help keep its shape during cooking. Sprinkle sea salt and black pepper evenly over a sheet of baking paper. Coat beef in olive oil and roll in the sea salt and pepper mixture until evenly coated. Sear beef in a large frypan over high heat until brown on all sides. Transfer to a baking paper lined tray.
Increase oven temperature to 200°C, transfer onions to the bottom shelf and place beef on the top shelf. Roast for 20-25 minutes or until beef is medium-rare and onions are tender and caramelised. Remove from oven and rest beef for 15 minutes before removing string and slicing into 4 thick slices.
Combine sour cream, milk, mustard and horseradish in a small bowl to make a dressing.
To serve, arrange Leafy Patch Watercress in the base of four wide serving bowls, top with a few onions, a slice of beef fillet and spoon over the dressing. Serve immediately.