It might surprise you to learn that yes, there are some lettuce varieties that you can freeze for a long time. However, this doesn't apply to every lettuce type. As a rule, thicker leaves freeze better than thinner ones which are fragile and break easily. Chop the heads of lettuce and put them in a mason jar, Ziploc bag, or plastic freezer bag secured with a rubber band, removing as much of the air as you can, then freeze it for up to 6 months. You can also puree the lettuce and keep it in ice cube trays. Once thawed, you can use the lettuce to make lettuce soup or as a garnish. The texture won't be as crisp as you'd expect with fresh lettuce, so I'd recommend only freezing lettuce if you have too much of it and it would otherwise be wasted.