1Grate zest from lemons to equal 1 tablespoon. Cut lemons in half; squeeze juice from lemons into a measuring cup to equal 1/2 cup.
2Whisk together half-and-half, sweetened condensed milk, and lemon juice. Pour mixture into freezer container of a 1 1/2 quart 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.