Step 1 Using a mortar and pestle, grind rosemary, garlic and zest together. When crushed, add honey and work into a paste. Spread over lamb, cover and refrigerate for a minimum of 4 hours (or overnight).
Step 2 Preheat oven to 190°C. Roast lamb for 20 minutes, then turn down the oven to 170°C and cook a further 1 1/2 hours, or 25 minutes per 500g meat.
Step 3 Meanwhile. make relish: Preheat grill to medium-high heat. Spray garlic and skins of capsicum with oil and place under grill. Cook until capsicum skins turn black and garlic is tender.
Step 4 Place capsicums into a clean, plastic bag to cool. (When cooled the skins will slip off easily.) Discard skins, then process capsicum in a food processor. Add garlic and tomatoes. Pulse to a coarse puree, then add balsamic vinegar and process to desired consistency. Cover and chill.
Step 5 When lamb is cooked, remove from the oven and rest for 15 minutes. Peel off the blackened crust and fat to reveal the meat underneath. Serve with relish.