On lightly floured surface, roll out pastry to 16- x 12-inch rectangle. Fit into 13- x 9-inch metal cake pan, draping overhang over sides. Brush mustard over bottom; set aside.
In bowl, whisk together ricotta, eggs, Parmesan cheese, all but 2 tbsp of the basil, and half each of the salt and pepper. Scrape into pastry shell.
Thinly slice tomatoes; arrange over filling, overlapping slightly. Fold overhang over filling. Whisk egg yolk with 1 tbsp water; brush over pastry.
Bake in centre of 400°F oven until pastry is puffed and golden and filling is set, 35 minutes. Sprinkle with remaining basil, salt and pepper. (Make-ahead: Let cool for 30 minutes. Refrigerate until cold. Cover and refrigerate for up to 4 hours). Cut into squares.