Step 1

Preheat oven to 250°F, with racks in upper and lower thirds. Stir together sugar and cornstarch. In the bowl of a mixer fitted with the whisk attachment, beat egg whites with lemon juice and salt on low speed until frothy. Increase speed to medium-high and gradually add sugar mixture, beating until stiff, glossy peaks form, 8 to 10 minutes.

Step 2

Draw two 5-by-10-inch rectangles on a sheet of parchment, spaced at least 2 inches apart. Turn parchment upside-down on a baking sheet and adhere corners with masking tape or tiny dollops of meringue. Repeat with another sheet of parchment and baking sheet, so you have 4 rectangles total. Divide meringue evenly among rectangles; spread to edges with an offset spatula.

Step 3

Bake until meringues are crisp and dry on outsides, but not developing any color, about 1 hour. Turn oven off (do not open door); let cool and continue drying in oven at least 2 hours and up to 1 day.

Step 4

Temper sorbets in refrigerator until softened but not melting, about 15 minutes. Transfer one meringue layer to a freezer-safe plate or tray. Gently spread one pint of sorbet evenly to edges of meringue. Repeat with remaining meringue and sorbet layers, topping with last meringue layer. Freeze until firm, at least 3 hours or, wrapped in plastic, up to 2 days.

Step 5

Beat cream to soft peaks on medium-high speed. Spread over top of cake; sprinkle with berries, coconut, and seeds. Slice and serve immediately, or cover with plastic wrap and return to freezer until ready to serve, up to 4 hours.