Pastry: In food processor, pulse flour, sugar and salt; pulse in butter and lard until mixture resembles coarse meal. Pulse in yolks; pulse in just enough ice water to make dough stick together. (Or, in bowl, whisk flour, sugar and salt; cut in butter and lard, then stir in yolks and water.) Shape into 2 thick discs. Cover and refrigerate for 1 hour.

Roll out 1 pastry disc to fit 10-inch/25 cm pie plate, leaving 3/4-inch/2 cm overhang.

Toss rhubarb, apples, sugar, cornstarch, cinnamon and salt; spoon into plate.

Roll out remaining pastry to scant 1/4-inch/5 mm thickness; cut into 14 1-inch/2.5 cm wide strips. Weave into lattice pattern over filling, leaving 1/4-inch/5 mm between each strip. Trim strips.

Whisk yolk with 1 tbsp/15 mL water; brush some over pastry overhang. Fold overhang up. Flute edge. Brush pastry with remaining egg wash; sprinkle with coarse sugar.

Bake in lowest third of 425°F/220°C oven for 20 minutes. Reduce heat to 350°F/180°C; bake until golden and bubbly, about 50 minutes longer (cover top with foil if browning too quickly). Let cool on rack.

Makes 8 to 10 servings.