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 tablespoonful at a time, until stiff peaks form. Beat in fennel seeds. Spread out meringue on a parchment-lined baking sheet into an 8″ round about 3/4″ thick in the center and 1 1/2″ thick at the edges, creating a well for the filling. Bake meringue until firm to the touch, about 3 hours. Turn off oven and let meringue cool in oven, about 2 hours. 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. Meanwhile, using an electric mixer, beat cream until soft peaks form. Beat in mascarpone just to blend. Carefully transfer meringue to a serving platter. Fill with whipped cream mixture. Top with berry mixture and tarragon. DO AHEAD: Meringue can be made 1 day ahead. Store airtight in a cool, dry place.