Simmer until cranberries have softened and released their juices, about 10 minutes. Strain mixture through a fine-mesh sieve into a medium bowl, pressing solids; discard solids. Add butter to cranberry mixture, whisking until combined. Let cool for 5 minutes. Add eggs and egg yolks, immediately whisking until well combined.
Return cranberry mixture to saucepan; cook over medium heat, whisking constantly, until thickened, 5 to 10 minutes. (Whisk should leave a faint trace when dragged through mixture).
Pour cranberry mixture into Tart Shell, smoothing top if necessary. Let cool completely on a wire rack. Chill, uncovered for at least 3 hours or up to 1 day. Remove tart from pan.
In a large mixing bowl set over a pot of simmering water, add egg whites, cream of tartar, and remaining ½ cup sugar. Stir constantly with a rubber spatula until mixture is warm to the touch and sugar is fully dissolved, about 10 minutes. Beat with a mixer on high speed until egg whites form stiff, glossy peaks and triple in size, about 8 minutes. Transfer meringue to a piping bag fitting with desired decorative tip. Pipe around edges of tart in desired pattern. Garnish with cranberries and ginger, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/cranberry-meringue-tart/