Very thinly slice lemon crosswise, using a handheld slicer and removing seeds with a knife tip as you go. Discard ends.
Toss lettuces in a large bowl with about 1/2 cup dressing. Add snap peas, radishes, and a little more dressing and toss again. Arrange salad on chilled plates and tuck in lemon slices and mint. Serve with more dressing if you like.