Place frozen raspberries and 1 Tbsp sugar into a small saucepan and cook over med-high, stirring often, until raspberries are broken down and syrupy (crush with a wooden spoon as you stir). Strain out seeds and cool completely. You should have approximately 1/4 cup of sauce.
Meringue:
Preheat oven to 300F and line a baking sheet with parchment or a silpat mat.
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.
Place bowl over a hot water bath on the stove and whisk constantly until the mixture is no longer grainy to the touch (approx. 3mins).
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch).
Sift cornstarch over meringue and whip until combined.
Reserve 1Tbsp of raspberry sauce and drizzle the rest over the meringue. Gently fold a few times to incorporate slightly. You want there to be steaks of raspberry.
Dollop onto prepared baking sheet, ensuring meringues are roughly the same size. I did all 8 on one sheet.
Drizzle remaining 1 Tbsp of raspberry sauce over meringues and swirl with a toothpick.
Bake for 25mins, turn oven off and leave meringues in there for 2 hours.