TO MAKE THE TART DOUGH:

1. In the bowl of a food processor, pulse together the all-purpose flour, powdered sugar, and salt until combined. Add the butter and pulse in short bursts until the butter is the size of small peas. Slowly add the beaten egg and pulse until the mixture comes together into a ball. Shape into a disk, wrap in plastic wrap and chill for at least 2 hours.
2. Let the dough sit at room temperature for a few minutes, then roll out to about 1/4 inch thick on a lightly floured surface. Use a large round cutter or a knife to cut out circles of dough for each tart pan (I used 5 inch circles for my 4-inch tart pans).
3. Grease tart pans with butter, then press the dough circles into each pan. Chill in the freezer until ready to bake.

TO MAKE THE FILLING:

1. In a medium saucepan, toss all ingredients together in a bowl to coat the fruit. Place over medium heat and cook, stirring frequently, until sugar is dissolved and mixture is moistened and the strawberry and rhubarb start to soften.
2. Divide filling between the 4 tart shells. Place on a sheet pan and bake at 400 degrees F until filling is bubbling and dough is golden, 15-20 minutes. Let cool completely, then remove tarts from their pans.

TO MAKE THE MERINGUE:

1. In the bowl of a stand mixer fitted with a whisk attachment, beat the egg whites until foamy. Slowly add the sugar to the egg whites while continuing to mix. Beat to stiff peaks. Spread meringue onto the tops of each tartlet.
2. Brown the meringue with a hand-held kitchen torch or under an oven broiler (watch it carefully). Serve immediately. Store any leftover tartlets in the fridge.