Cook over low heat until steaming, whisking constantly. Raise the heat to medium-low and continue cooking, whisking constantly, until the mixture thickens and starts to bubble. Once the bubbles appear, continue whisking and cooking for two full minutes (set a timer! It’s important to not skimp on the time or the filling will not set properly). Take care as the mixture will sputter and spit a bit.
Strain the filling into the prepared container to remove the zest, then scrape the filling over the prepared crust. Cool at room temperature until a skin forms over the surface, about 30-45 minutes, then cover with plastic wrap and refrigerate until the filling is cold and fully set, at least 4 or up to 24 hours.
Make the strawberry meringue and assemble the bars: About an hour before serving, remove the bars from the refrigerator and transfer to a serving plate. Discard the parchment.
To make the meringue, fill a medium saucepan with 2-3 inches of water and bring to a simmer. While the water is heating up, combine the freeze-dried strawberries and sugar in the bowl of a food processor. Pulse until the strawberries are ground into a fine powder. Transfer the strawberry-sugar to the bowl of a stand mixer and whisk in the egg whites, cream of tartar, and salt.
Place the bowl on top of the saucepan to create a double-boiler—this heats the egg mixture gently to avoid scrambling the eggs. The base of the bowl should not touch the simmering water.
Heat the egg white mixture, stirring frequently and scraping the sides and bottom of the bowl with a heatproof spatula, until it reaches 165°F (74C) on an instant-read thermometer. The mixture should be quite thick and the sugar completely dissolved.
Remove the bowl from the double-boiler and transfer to a stand mixer fitted with the whisk attachment. Whisk on high speed until the meringue has increased in size 3-4x times and holds glossy medium-stiff peaks, about 4-5 minutes. (It will still be a little warm — don’t overbeat or the meringue can get a little gooey and difficult to spread.)
Immediately scrape the meringue on top of the lemon filling and use a spatula or spoon to spread it over the top. Swoop and swirl as you desire.
To serve, use a hot, sharp knife to slice (clean the knife with a hot, damp towel between cuts). The bars are best served immediately. Refrigerate any leftovers in an airtight container.
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