Muskmelon is like cantaloupe, and it’s great with the lime cream in this tart. As for getting the melon into that “geometric state of mind,” it’s easy. Use an apple corer to create a perfect column of melon, cut triangles from the flesh, scoop out circles. Then arrange the fruit on the tarts in random sculptural form. There is no right or wrong way, only what’s pleasing to your eye.