Combine rice, 1 tsp sea salt and 2½ cups (625ml) water in a large saucepan over high heat. Bring to the boil, reduce heat to low. Cover and cook for 20 minutes until rice is tender and water is absorbed. Set aside, covered, for 10 minutes to steam.
Combine mirin and 1/4 cup (60ml) vinegar in a bowl. Transfer rice to a large bowl. Use a wooden spoon or rubber spatula to break up lumps while gradually adding mirin mixture, gently folding to combine well.
Lightly oil a 20cm square cake tin and line with plastic wrap, allowing it to overhang the sides by a few centimetres (this will make it easier to lift the rice from the tin). Press rice mixture evenly into the prepared tin with wet hands, smoothing the top. Cover with plastic wrap, then op with a slightly smaller square cake tin. Place cans on top and refrigerate for 2 hours or overnight to set.
To make the dipping sauce, whisk ingredients in a small bowl to combine.
To serve, use the plastic wrap to lift the rice onto a cutting board. Use a large spoon to scoop each Hass avocado half from its skin in a single piece, then thinly slice lengthways. Arrange on top of the rice, then use a large sharp knife to cut into fingers (if you dip the knife into water between each cut it will make the rice easier to slice). Scatter with sesame seeds and spring onion and serve with miso dipping sauce.