Preheat oven to 200°C. Tie the beef with short pieces of string at 5cm intervals (you can also get your butcher to do this). Brush the beef lightly with oil and sear all over in a medium-high non-stick fry pan until evenly browned. Season and brush the combined garlic, thyme and horseradish cream over beef.
Place the beef in a roasting dish, toss the onions around the meat, coating lightly in a little oil. Roast for 45-60 minutes for rare, 60-75 minutes for medium, or 75-90 minutes for well done. For ease and accuracy use a meat thermometer.
Remove beef, cover it loosely with foil, and rest it in a warm place for 10-20 minutes before carving across the grain.
Meanwhile, heat a lightly oiled grill pan or barbecue over high heat. Cook the eggplant, pumpkin and mushrooms for 2 to 3 minutes each side or until charred and tender.
Whisk the tahini, honey and lemon juice in small bowl to combine. Season with salt and pepper.
Serve the beef with the onions, char-grilled vegetables sprinkled with sesame seeds, tahini dressing and green beans.