Fill a large bowl with cold water. Add vinegar and flour and mix. Brush black salsifies vigorously under running cold water, peel and immediately place in the prepared flour vinegar water.
Wash the lemon thoroughly, grate dry and finely grate the peel. Halve the lemon and squeeze it. Boil up salted water in a large pot. Add 2 tablespoons lemon juice and black salsify, cover and cook for 15-20 minutes until firm to the bite.
For the sauce, melt butter at medium heat, bring to the boil and skim off the foam with a ladle. Remove from the heat and let it cool down. Wash tarragon, pluck leaves, except for a few for garnishing, from the stalks and chop finely.
Beat egg yolks, 1 tbsp. lemon juice and white wine over a hot water bath until frothy. Gradually fold in the liquid butter, season to taste with the remaining lemon juice, sugar, salt and pepper. Stir in tarragon and lemon peel.
Lift the black salsifies out of the stock, allow to cool slightly. Cut the ham in half lengthwise. Wrap 2 slices of ham around 5-7 black salsifies and place on ovenproof plates. Pour the hollandaise sauce over them and grill under the hot grill of the oven for 2-4 minutes until golden brown.
Remove, let cool briefly, garnish with the remaining tarragon. Serve with a rustic farmhouse bread.