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.
In light of the recent product recalls, we want to reassure you that Tripod Farmers, including our brands Bacchus Marsh and Leafy Patch, are NOT affected. Thank you for your trust — we look forward to continuing to serve you fresh, high-quality produce.