These delightful tarts offer a burst of citrus freshness that's welcome after a big holiday meal. When you start with purchased pie shells, they come together quickly.
Combine lime zest, juice, gelatin and 1/4 cup cold water in a small saucepan. Heat over medium heat until gelatin is dissolved, stirring, about 3 minutes. Pour into a large bowl and let cool about 10 minutes.
Pour heavy cream into a second large bowl. Beat with an electric mixer until medium-stiff peaks are formed. Whisk by hand to stiff peaks.
Add sweetened condensed milk to gelatin mixture; beat until well combined. Fold in whipped cream and then divide evenly among prepared crusts. Refrigerate at least 2 hours, or until firm. Garnish with additional whipped cream and lime zest, if desired.