Preheat oven to 250ºF. Line a rimless baking sheet with parchment paper. Use an overturned 9" cake pan to trace a circle on the parchment, then flip it over so the pencil side is facing down on the baking sheet. Set aside.
Combine egg whites and cream of tartar in the bowl of a stand mixer. Start beating on medium-low until egg whites froth, then increase to medium-high and beat until soft peaks form.
With the mixer running, add the sugar in a very slow stream (a tablespoon at a time) until glossy, stiff peaks form. Beat in the cornstarch (optional: sift it if it's clumpy), vanilla extract and salt.
Spoon the meringue into a large pastry bag fitted with a big round tip. (It's helpful to place the pastry bag inside a tall glass to hold it upright.)
Hold the pastry bag upright and squeeze to create a mound of meringue. Continue making mounds all around the circle that you traced, to form a wreath. You can always go back and add more meringue to any mound, so don't worry about making them too small or uneven.
Once you've piped mounds all the way around, use the back of a spoon to create a well in each mound (that's where you'll put the whipped cream and pie filling).
Bake for 1 hour, then turn off the oven and let it cool inside the oven for 1 hour. The outside will be dry and pale, and the inside will be marshmallowy and soft.
Just before serving, whip the cream to soft peaks. Whisk in the vanilla. Spoon the whipped cream into the dips you made in the meringue. Top with spoonfuls of cherry pie filling. Garnish with rosemary sprigs and pomegranate arils if desired.