1. Grate zest from lemons to equal 1 Tbsp. Cut lemons in half; squeeze juice from lemons into a measuring cup to equal 1/2 cup.
2. Whisk together half-and-half, sweetened condensed milk, and lemon juice. Pour mixture into freezer container of a 1 1/2-qt. electric ice-cream maker, and freeze according to manufacturer's instructions. (Instructions and times may vary.) Stir in graham cracker crumbs and lemon zest; transfer to an airtight container. Freeze 2 hours before serving.