Preheat oven to 350°. Scatter sugar in a shallow baking dish and bake 10 minutes. After 8 minutes, using an electric mixer on medium-high speed, beat egg whites and cream of tartar in a large bowl until foamy.
Remove sugar from oven and decrease temperature to 200°. With the mixer running, gradually stream sugar into egg whites. Add salt and beat until stiff peaks form and bowl is barely warm to the touch. (Heads up: This can take as much as 10 minutes with a stand mixer and up to 20 minutes with a handheld mixer.) Scrape in vanilla seeds, reserve pod for another use, and beat just to combine.
Scoop large spoonfuls of meringue onto 2 parchment-lined baking sheets to make 12 mounds. Make a slight indentation in the center of each mound with the back of the spoon, pushing out to create 3"-diameter rounds. Top the border of meringues with nuts and/or poppy seeds, as desired.
Bake meringues until dry and firm, about 2 hours and 15 minutes. Turn off oven and use a wooden spoon to prop door ajar. Let meringues cool completely in oven.
Do Ahead: Meringues can be made 1 day ahead. Store tightly wrapped at room temperature.
Combine sugar and 2 cups berries in a medium bowl and mash together with a fork until sugar is dissolved and mixture is bright in color and pourable.
Whisk cream in a medium bowl to medium peaks. Spoon whipped cream into centers of meringues, top with remaining berries, then drizzle with sauce.
Do Ahead: Berry sauce can be made 4 hours ahead. Store tightly covered at room temperature.