Put lemon juice and water in a bowl. Wash the salsify, peel them and immediately place them in the lemon water so that the roots do not turn brown. Cut the salsify into pieces and put them back in the water.
Sauté onions and garlic in butter, add black salsify, dust with flour and stir well. Deglaze with the white wine and pour in the broth. Let everything simmer for about 40 minutes. Add the cream and puree finely with the hand blender. Bring to the boil again briefly, remove the saucepan from the plate and let the Roquefort melt in it. Season with pepper, nutmeg and salt.