Make the crust: Pulse the flour, granulated sugar and salt in a food processor to combine. Add the shortening and pulse until the mixture looks like coarse meal. Add the butter and pulse a few times until it is in pea-size pieces. Add the egg yolk and ½ cup ice water and pulse a few times until the dough just begins to clump together; add up to ¼ cup more water, 1 tablespoon at a time, as needed. Lay out 2 sheets of plastic wrap; put about three-quarters of the dough on one sheet and the remaining dough on the other. Gently pat each into a rectangle, using the plastic wrap to help you. Wrap tightly and refrigerate until firm, at least 2 hours and up to 24 hours.
Roll out the larger piece of dough into a 13-by-19-inch rectangle (about ¿ inch thick) on a lightly floured surface. Ease into an 11-by-17-inch jelly roll pan, leaving a 1-inch overhang.
Make the filling: Pulse the nuts in a food processor until roughly chopped. Combine with the chocolate chunks in a bowl, then evenly sprinkle the mixture over the crust. Whisk the melted butter, eggs, brown sugar, corn syrup, lemon juice and vanilla in a bowl; pour over the nut mixture.
Roll out the smaller piece of dough into a 10-by-12-inch rectangle on a lightly floured surface; cut out ten 1-inch-wide strips of dough. Lay 5 strips diagonally over the filling, about 2 inches apart, then lay the remaining strips perpendicular to them. Trim the strips at the edge of the baking sheet (use the trimmings to extend any strips that are too short). Fold in the overhang and crimp the edges with your fingers. Brush the edges and the strips of dough with heavy cream. Refrigerate until firm, about 30 minutes.
Preheat the oven to 375 degrees F. Bake until the crust is golden and the filling is set, 40 to 45 minutes. Transfer to a rack and let cool completely, at least 2 hours.