Make green oil: chop tarragon, put in a blender, pour in grape seed oil, lime juice and 1 tsp. sugar, beat until smooth, strain through cheesecloth. Crush a clove of garlic into this oil, let it brew for 2-3 hours and strain again.
Put the nelma fillet in the freezer for 20-30 minutes.
For “caviar”, put currants in a saucepan, add 200 g of water and 60 g of sugar, bring to a boil, beat with a blender, wipe through a sieve. Add agar-agar to the resulting mass and let it dissolve.
Pour ice water into a large bowl with a diameter of more than 20 cm. Draw warm currant sauce into a syringe and drop it drop by drop into the water - passing through a layer of ice, already formed “eggs” will sink to the bottom. Strain the finished "caviar" through a sieve, dry it - it should be of a consistency slightly denser than marmalade.
Remove the core from the red onion, cut into quarters, pour in vinegar and leave for 10 minutes, then dry.
Cut the nelma into the thinnest (1-2 mm) slices, put on plates. Put sorrel leaves on the fish, pour over with green oil, garnish with red onion and currant "caviar", salt and pepper.