Tuna sushi rolls

Sushi rice

• 400 g sushi rice
• 510 g water, plus extra to soak
• 60 g rice wine vinegar
• 20 g sugar
• 10 g sea salt

Filling

• 4 nori sheets
• 185 g canned tuna in oil, drained and flaked
• 2 tbsp Kewpie mayonnaise
• 1 continental cucumber, cut lengthway into 1 cm thick strips
• 1 avocado, flesh only, sliced