Scrape out and discard flesh of preserved lemon. Rinse peel and put in a blender or food processor.
Add olive oil, lemon juice, parmesan, pepper, and a pinch of salt to taste. Blend until emulsified. Dip a leaf into dressing to taste; add more salt or lemon juice if you like.
Put lettuces in a large bowl or on a platter and drizzle with dressing. Toss gently to coat. Add about 1/4 cup shaved parmesan and toss gently once more.