Whisk to combine the flour, sugar, and salt. Using the rubbing method or a pastry cutter, cut the butter into the flour mixture until it resembles breadcrumbs.
Add the egg, and using a rubber spatula, mix the egg into the flour. Chill the dough for 30 minutes. Roll the dough out to a ⅛" thickness.
Transfer it to a 9" tart pan. Chill for another 30 minutes. Line the pastry with parchment paper and fill with pie weight or dried beans or lentils. Bake for 30 minutes.
Place the rhubarb, sugar, salt, and a 1/4 cup of the water in a small saucepan. Stir the cornstarch mixture into the simmering rhubarb. Let cool.
Take the pastry cream off of the heat and add the butter. Stir until the butter melts. Pour the cream into the tart shell and add the stewed rhubarb on top. Let chill for 3 hours.
While the sugar mixture is heating up, whisk the egg whites and cream of tartar on high using a hand mixer or a stand mixer until soft peaks form.
With the mixer running, stream the hot sugar mixture into the egg whites. Continue to whisk until stiff peaks form.
Place the Strawberry Meringue on top of the set Rhubarb Custard Tart. Garnish with lilac blossoms or candied violets, if desired.