Tip: To make tarts, shape one-third of the pastry into a cube. Divide remaining pastry into 4 discs. Wrap each with plastic and chill for 30 min. Roll discs into 7-in. circles. Press pastry into 4 4-in. tart pans (1 1/4 in. deep) set on a baking sheet, leaving 1-in. overhang. Brush overhang with egg wash. Divide rhubarb mixture among pastry shells. Roll out pastry cube into a 10-in. square, about 1/8 in. thick. Cut in half, making 2 rectangles, then cut each piece crosswise into 12 3/4-in. strips, creating 24 strips 5 in. long. Lay 3 strips vertically across each tart. Follow instructions for lattice top here. Bake in lower third of oven until golden brown, 40 to 50 min. Transfer to a rack to cool completely.