Slice the beetroot thinly using a mandoline and set aside.
Boil 400ml water with sugar, vinegar, and salt in a pan.
Divide the pickling liquid into two separate pans.
Submerge beetroot in one pan and remaining vegetables in the other; leave to cool.
Season 400ml beetroot juice with salt and pepper in a pan.
Whisk in xanthan gum over gentle heat until the mixture thickens.
Mix apricot jam with 100ml beetroot juice and coat the tuna steaks.
Heat oil in a large frying pan until hot.
Cook tuna steaks for 1 to 2 minutes on each side.
Rest the tuna for a few minutes before slicing.
Brush purée onto plates, top with tuna, pickled vegetables, and micro herbs.