This is the perfect make ahead dessert; all three components can be prepped the day before and assembled just before serving. Leftover maple syrup will be divine on pancakes or waffles at a holiday brunch.
Preheat oven to 225°F. Line a baking sheet with parchment paper; secure paper to baking sheet with masking tape.
Whisk together granulated sugar and cornstarch in a medium bowl; set aside. Place egg whites in the bowl of a heavy-duty electric stand mixer fitted with the whisk attachment. Beat on medium speed until foamy, about 1 minute. Add cream of tartar and salt; beat until blended. Add sugar mixture, 2 tablespoons at a time, beating until glossy, stiff peaks form, and sugar dissolves, about 3 minutes. (Do not overbeat.)
Gently spoon egg white mixture into 8 (3-inch-wide x 2 1/2-inch-tall) mounds on prepared baking sheet. Bake on middle rack in preheated oven until meringues have formed a crust, about 1 hour and 30 minutes. Turn oven off; let meringues stand in oven with door closed at least 4 hours and up to 12 hours. Meanwhile, place maple syrup in a microwavable bowl, and microwave on high until warm, about 1 minute. Stir cranberries into maple syrup; cover and refrigerate 4 hours or up to overnight.
Place yogurt in a medium bowl. Using a rubber spatula, fold mascarpone, zest, and rosemary into yogurt until smooth; set aside. Strain maple syrup and cranberries; place cranberries in a medium bowl, and reserve maple syrup for another use. Add apple slices and pomegranate seeds to cranberries; toss to combine.
To serve, spoon 1 1/2 tablespoons yogurt mixture onto each meringue, and top with about 1/3 cup fruit mixture; garnish each with a rosemary sprig.