Follow package instructions to fit crust into a 9-inch tart pan with removable bottom, and bake until light golden.
Make pastry cream: Mix flour, cornstarch and sugar in a bowl. Add 1/3 cup milk; beat until smooth. Whisk in yolks. In a pan, bring remaining milk to a simmer. Whisking constantly, pour hot milk into yolk mixture. Pour back into pan and bring to a boil, stirring. Boil for 2 minutes, whisking. Remove from heat; beat in butter and vanilla. Pour into a bowl; press plastic wrap directly onto surface of pastry cream. Chill 1 hour.
Finish tart: Stir jam, lemon juice and 2 Tbsp. water in a pan over low heat until jam melts; strain. Remove crust from tart pan; brush with jam glaze. Spread pastry cream evenly. Arrange raspberries in center, forming a star. Fill in remaining surface with blueberries. Brush tart with remaining glaze.