Method

• 1.
  For the onion and horseradish jam, heat oil in a large frypan over high heat. Cook onion, stirring occasionally, for 5 minutes. Season. Reduce heat to low and cook, stirring occasionally, for 10 minutes until onion is softened and well browned. Add sugar and vinegar, increase heat to high and cook, stirring, for 2-3 minutes until liquid evaporates.
• 2.
  Transfer to a medium bowl and set aside for 10 minutes to cool slightly. Stir in horseradish until combined. Chill until ready to use.
• 3.
  Season beef with freshly ground black pepper. Heat oil in a large frypan over high heat. Cook beef, turning, for 8 minutes until browned on all sides. Set aside to cool.
• 4.
  Blanch spinach leaves in a saucepan of boiling water for a few seconds. Cool immediately in iced water. Drain well and pat dry with paper towel.
• 5.
  Preheat oven to 200°C. On a sheet of baking paper, arrange prosciutto, overlapping, to form a 25cm x 30cm rectangle (adjust width of rectangle to fit length of your piece of beef). Top with spinach. Spread 1/4 cup onion jam over beef. Place beef on 1 short side of prosciutto and spinach. Roll tightly to enclose, using the paper as a guide.
• 6.
  Place roll on a wire rack on a baking tray. Roast for 1 hour for medium-rare or until cooked to your liking. Set aside, loosely covered, for 20 minutes to rest. Serve with remaining onion jam alongside.