Place the egg whites and cream of tartar in the bowl of a mixer fitted with the whisk attachment (or in a bowl with a hand mixer). Mix on high until the egg whites are foamy.
Continue to mix on high speed while gradually adding the sugar. Whip until the egg whites form stiff peaks. Do not underbeat.
Line two baking sheets with parchment paper. Using a 3-inch circular cookie cutter (or edge of a glass), trace 12 circles onto the parchment paper. Turn over the parchment paper so that the pencil marks are on the underside of the paper. They should still be visible from the top side.
Divide the egg white mixture evenly between the 12 circles, using the back of a small spoon to make a shallow well in the middle of each circle.
Bake for 1 hour, then turn off the oven and leave the meringues in the oven for an additional 1 ½ hours so that the meringues dry completely.
Gently peel the meringues off of the parchment. Cool completely. Set aside 6 meringues for this dessert and freeze the remaining 6 meringues in an air-tight container for future use.
In a medium bowl, whisk together the yogurt and agave nectar (or honey) until combined. Divide the mixture evenly between the meringues.
Top each filled meringue with pomegranate arils and pistachios. Serve.