To make the dressing,place all the ingredients and one tablespoon of water in a bowl and whisk well to combine. Season to taste with white pepper and set aside.
Season the tuna with sea salt and white pepper to taste.
Heat oil in a heavy-based frying pan over medium-high heat until just smoking and sear fish for 15 seconds on each side,so it's lightly cooked on the outside but still raw in the middle. Set fish aside to rest.
Divide the salad leaves among four plates,then place the avocado on the leaves and add the shallots.
With a sharp knife,cut the fish into thin,even slices. Place the fish on the salad and spoon the dressing liberally over the fish and leaves. Sprinkle with extra toasted sesame seeds (if desired). Season the fish with salt and a grind of white pepper to taste,then serve.