Step 1

Preheat oven to 200°. Using an electric mixer, beat egg whites, cornstarch, and cream of tartar in a large bowl until frothy. Gradually beat in 1/2 cup sugar, 1 table-spoonful at a time, until stiff peaks form. Beat in fennel seeds.

Step 2

Spread out meringue on a parchment-lined baking sheet into an 8-inch round about 3/4 inch thick in the center and 1 1/2 inches thick at the edges, creating a well for the filling.

Step 3

Bake meringue until firm to the touch, about 3 hours. Turn off oven and let meringue cool in oven, about 2 hours.

Step 4

About 30 minutes before serving, toss strawberries, raspberries, lemon zest and juice, and remaining 1/3 cup sugar in a large bowl; let stand to let fruit release juices.

Step 5

Meanwhile, using an electric mixer, beat cream until soft peaks form. Beat in mascarpone just to blend.

Step 6

Carefully transfer meringue to a serving platter. Fill with whipped cream mixture. Top with berry mixture and tarragon.

Step 7

DO AHEAD: Meringue can be made 1 day ahead. Store airtight in a cool, dry place.