single-crust plum and apple pie

INGREDIENTS

• 7 tablespoons (100 grams) unsalted butter, at room temperature
• 1/2 cup (100 grams) granulated sugar
• 1/2 teaspoon finely grated orange zest
• 1 large egg, lightly beaten
• 1 cup plus 6 1/2 tablespoons (175 grams) all-purpose flour
• 1/2 teaspoon baking powder
• 1/4 teaspoon coarse or flaky salt (or less of table salt)
• Milk or cream, for brushing crust
• Coarse or fine sugar, for sprinkling crust
• Softly whipped, lightly sweetened cream, for serving
• 1 pound ripe prune plums or greengages, halved, pitted, and halved again (i.e. quartered)
• 1 pound apples, peeled, cored and cut into smaller chunks (than you see in my photos)
• 2 tablespoons sugar
• 1/8 teaspoon ground cinnamon
• Squeeze or two of orange or lemon juice