01 - Preheat oven to 350°F.
02 - Mix graham cracker crumbs, melted butter, and sugar in a bowl until the mixture resembles wet sand.
03 - Press the mixture evenly into the bottoms and sides of 6 mini tart pans.
04 - Bake for 8-10 minutes. Allow to cool completely.
05 - Purée watermelon, lime juice, and honey in a blender until smooth. Strain to remove any pulp.
06 - Beat cream cheese until smooth. Add sweetened condensed milk, heavy cream, lemon zest, and lemon juice. Beat until light and creamy.
07 - Pour the lemon cream into the cooled tart shells, spreading evenly.
08 - Spoon a thin layer of watermelon purée over the lemon cream and lightly swirl with a skewer to create a marbled effect.
09 - Freeze tarts for at least 3 hours or until completely firm.
10 - Let tarts sit at room temperature for 10 minutes to soften slightly before serving. Garnish with mint, watermelon balls, and extra lemon zest if desired.