1. Step 1

   Combine mustard, peppercorns, rosemary, sugar and 1/4 cup beer in a bowl (see Note). Crush 1/2 the garlic cloves and add to beer mixture.

2. Step 2

   Tie beef with kitchen string at 5cm intervals. Rub all over with beer mixture. Refrigerate for 2 hours or overnight, if time permits.

3. Step 3

   Preheat oven to 200C/180C fan-forced. Place beef, including marinade, in a roasting pan. Add stock, and remaining beer and garlic to pan. Roast for 1 hour for medium, or until cooked to your liking. Transfer garlic cloves to a small bowl. Cover beef loosely with foil. Stand for 20 minutes. Meanwhile, using a fork, mash garlic cloves until smooth. Add butter. Season with salt and pepper. Mash until smooth.

4. Step 4

   Remove and discard string from beef. Slice beef. Serve with garlic butter and pan juices, if desired.

Store remaining beer in an airtight bottle or container if marinating overnight.