Soak kombu in water for 1 hour
Wrap kombu around fish and glad wrap. Leave in fridge for 1 hour
Blanch shallots for 30 seconds, then BBQ until nicely charred. Slice in half
Slice radish on a mandolin (1mm thick)
In a frying pan, toast buckwheat on low heat until golden
Refresh sorrel in ice water and leave to dry, being careful not to bruise the leaves
Combine all ingredients and mix well
Slice tuna into 4 x 25g pieces and place overlapping
Layer sorrel next to tuna and weave in watermelon radish slices & shallots
Sprinkle salt and toasted buckwheat over tuna
Lightly drizzle dressing over entire dish