I used to LOVE to make pies—double crust, complicated beautiful pies. And then I had kids. The kids are eager bakers and always want to help, so I had to give up on perfection and find a new way to enjoy pie making that isn't stressful for me or them. Skillet pies are a perfect solution—they're very forgiving, and since you only need one crust to top the pie, I can use my standard pie dough and make two crusts—one for baking now and one to freeze or refrigerate for later in the week. Or, for company, I bake two skillet pies, side by side, at once. This crust uses rye flour for a crunchier, nuttier and slightly more rustic finish. It makes the dough super tasty but a bit fragile. For the filling, our favorite is a mix of apples and plums—just enough sweet apples and tart, juicy plums to give this pie filling a bubbly ruby color and a great tart flavor. I always use a combination of apples, and slice them thinly, rather than in thick chunks, so the plums and apples sort of melt together.
For the crust: Place the butter in the freezer while you measure the dry ingredients. Place the water over ice (you will drain it later) or into the freezer to chill.
Stir together the flours, sugar and salt in a medium bowl. Use the coarse side of a box grater to grate the butter all over the flour. Toss together with your fingers, pinching to break some of the butter into smaller bits and to coat with flour, working quickly to keep the butter cold. Continue rubbing until you have some pea-sized pieces of butter, with some slightly larger knobs. Add the vinegar, drain the water (if over ice) and add, a few tablespoons at a time, stirring with a fork to make a shaggy dough that mostly holds together (a food processor works well for this, too!). Squeeze the dough to form a ball, adding more ice water, 1 tablespoon at a time, to hold it together if needed.
Divide the dough in half and place each half on a generous sheet of plastic wrap. Sprinkle lightly with water if needed (if still dry and crumbly). Use the edges of the plastic wrap to pull and press the dough into a nice round disk and wrap tightly. Repeat with the remaining dough. If using right away, chill the dough in the refrigerator at least 1 hour. Otherwise, refrigerate up to 4 days or freeze up to 1 month.
For the filling: Toss together the plums, apples, sugar, cornstarch, lemon juice, salt and cinnamon in a large bowl and mix well. Butter a cast-iron (or other oven-proof) skillet generously with butter and transfer the filling to the skillet, taking care to scrape any juices into the pan. Dot the top of the fruit with small pieces of the butter. Set aside (or refrigerate for up to 1 hour while you prepare the dough; don't leave too long since the lemon can react with the cast iron).
Meanwhile, preheat the oven to 425 degrees F.
Remove one disk of the dough from the fridge and roll on a lightly floured surface, dusting with flour as needed to prevent sticking. Roll the dough into an 1/8-inch-thick round. Make sure the dough isn't sticking to the surface, then use a knife, pizza cutter or pastry scraper to cut the dough into 10 thick strips, each about 1 1/2 inches wide.
Arrange the strips over the fruit in a crosshatch pattern: First, lay 3 strips across the top of the pie in the same direction. Pull the middle strip back slightly, then overlap with a strip going the opposite direction. Repeat, lifting the strips as needed to weave the perpendicular strip up and over. (The rye flour makes the dough flaky but delicate, so once the strips are on the fruit, don't fuss with them much. Don't worry if anything tears—you can press it together and use the egg wash to repair later.) Weave any extra dough around the outside rim of the skillet and press together, using cookie cutters or your fingers to make a design.
Whisk together the beaten egg and cream. Brush the egg wash all over the top, using it to help repair any rips or tears. Sprinkle lightly with sugar. Bake the skillet pie for about 15 minutes. Lower the temperature to 250 degrees F and continue to bake until the filling is bubbly and the crust is evenly golden brown, another 40 to 50 minutes. Cool slightly before serving, or cool completely. Cut into eight pieces. Spoon into shallow bowls and serve with ice cream.