Little Gem lettuce is a small, sweet romaine-type variety. Romaine hearts can be used in its place. Recipe courtesy of Clare Vivier.
Whisk together both vinegars, shallot, and mustard. Gradually whisk in both oils; season with salt and pepper. Stir in walnuts.
Arrange lettuce on a large platter; spoon vinaigrette evenly over top and serve.