2 tsp. kosher salt
12 (1-inch-thick) slices from a 5-pound quartered chilled watermelon
6 lime wedges
Heat a dry small heavy skillet over moderately low heat until hot, about 3 minutes, then toast fennel seeds, stirring constantly, until lightly browned, 3 to 4 minutes. Transfer seeds to a work surface and cool 10 minutes. Coarsely crush seeds using bottom of skillet, then stir together with salt and sprinkle over watermelon. Serve with lime wedges.