Frozen Watermelon-Lime Bars

For Watermelon Sorbet:
1 (2 1/2-pounds) piece of watermelon
1/2 cup sugar
2 tablespoons fresh lime juice
1 tablespoon tequila

For Lime Semifreddo:
1 (14-ounce) can sweetened condensed milk
1 teaspoon grated lime zest
1/2 cup fresh lime juice
1/2 cup chilled heavy cream

Equipment:
An ice cream maker

Make watermelon sorbet:
Line a 9-inch square baking pan (2 inches deep) with plastic wrap or parchment paper, leaving a 2-inch overhang. Put lined pan in freezer.

Coarsely chop watermelon flesh (leave seeds in), then purée enough to yield 2 1/2 cups in a blender.

Add sugar, juice, and tequila to purée and blend 30 seconds.

Freeze sorbet in ice cream maker. Transfer to lined baking pan, smoothing top. Put in freezer to harden, at least 1 hour.

Make lime semifreddo:
Whisk together condensed milk, zest, and juice. Beat cream until it just holds stiff peaks, then gently fold into condensed-milk mixture.

Spread over sorbet, smoothing top. Freeze until solid, at least 2 hours.

To serve, lift dessert from pan using plastic wrap. Cut into 12 bars and serve on chilled plates.