Place the apple juice in a pan with the mustard seeds, sugar, vinegar and salt. Bring to the boil over a high heat and boil unti reduced by half.
Add the oil and whisk in well. Season with pepper and allow to cool.
Peel the onions and wash the radishes. Cut or shave both iinto thin rings or slices.
Slice the cheese and place in a bowl with the onions and radishes. Add the cooled dressing and stand for 10 minutes.
Meanwhile, wash the chives, shake dry, finely chop and scatter over the salad. Serve with lettuce.