Dissolve cornstarch in 3 Tbsp. water in a small bowl; set aside. Combine strawberries, rhubarb, sugar and lime in a large heavy saucepan. Cook over medium heat, stirring often, until sugar dissolves and juices are released, about 4 minutes. Stir in cornstarch mixture and bring to a boil. Transfer to a bowl. Chill until cool, about 30 minutes.
Preheat oven to 375°. Roll out dough on floured parchment paper to 12″ round; brush with beaten egg. Dust the center circle with a mixture of 1T flour, and 1T sugar – will prevent the crostata from having a “soggybottom”.
Mound filling in center of crust; gently spread out, leaving 1 1/2″ border. Gently fold edges of dough over filling, pleating as needed. Brush border with egg; sprinkle with raw sugar.
The crostata can be immediately baked, or frozen and baked from frozen so the butter is nice and cold (my preference).
Bake on a sheet tray on bottom rack for 25mins-30 min and rotate. Checking in every 10 minute or so, bake until the fruit is juicy and bubbling, and the crust is golden, about 45 min total. Cool completely before slicing. Transfer crostata to a platter, cut into wedges, and serve with whipped cream or ice cream.