Trim rootlets from mâche. Gently dunk leaves up and down in lukewarm water, then lift out. Repeat until no grit remains. Pat dry on soft, thick towel. Separate Bibb lettuce leaves, rinse, and spin-dry, then cut to suit. Combine mâche and Bibb in a serving bowl. Do not refrigerate.
In a small bowl, combine lemon juice, salt, sugar, nutmeg, and tarragon. Combine