Step 1 Rinse rice under cold water until water runs clear. Drain. Place rice and 6 cups cold water in a heavy-based medium saucepan over medium-high heat. Bring to the boil. Remove from heat. Set aside (without lifting lid) for 10 minutes.
Step 2 Spread rice out over a large glass baking dish or tray. Drizzle vinegar over rice. Slice through rice with a spatula to combine. Cover with a damp tea towel to prevent drying out. Set aside to cool completely.
Step 3 Combine the tuna, mayonnaise, half the juice and garlic in a bowl. Taste and season with freshly ground black pepper. Drizzle avocado with remaining lime juice to prevent discolouring.
Step 4 Place a nori sheet, shiny side down, on a clean, flat surface. Working from the short end, spread a quarter of the rice over the nori, leaving 2cm free at the top end. Place some avocado, capsicum and tuna mixture down centre of rice. Brush the top 2cm of nori with water. Working from the opposite end, roll up the nori to enclose the filling. Repeat with the remaining ingredients to make 4 rolls. Cut in half. Serve with soy sauce and wasabi.