Halve the fennel lengthwise. Cut out the small triangular core, trim the ends and discard. Slice the remaining fennel as thinly as possible.
Trim the lettuce hearts. Halve lengthways, halve again and cut each quarter into half. Rinse and shake dry.
To make the dressing, in a large bowl, whisk together the vinegar, lemon juice, mustard and olive oil. Season with salt, pepper and