Heat oven to 400°F. Line a 10x15-inch rimmed baking sheet with parchment.
Lightly flour the counter. Roll half the dough to a rectangle slightly larger than the pan.
Fit the dough into the pan with a small overhang. Chill while you prepare the filling.
Combine strawberries, sugar, cornstarch, lemon juice, lemon zest, vanilla, salt, and cinnamon. Toss and rest 10 minutes.
Spoon the strawberry mixture into the crust and spread evenly. Dot with butter pieces.
Roll the remaining dough to a rectangle to cover the filling. Place over the top and trim excess.
Fold edges together and crimp to seal. Cut several vents on top for steam to escape.
Brush the crust with milk and sprinkle with coarse sugar.
Bake 30–35 minutes, until the crust is deep golden and juices bubble through the vents.
Cool on a rack at least 1 hour to set the filling before slicing.