6tablespoonscold, unsalted butter, in 1/4" dice(3 ounces / 3/4 stick)
1 - 2tablespoonsice water
For brushing the crust:
1tablespooncream or milk
1tablespoonsugar (such as turbinado)
4large apples (see headnote), peeled, cut off the core, and sliced 1/2" thick(1 1/2 pounds)
1 1/4poundsrhubarb (about 5 medium-large stalks), halved lengthwise if fatter than 1", sliced 1/2" thick
1/2cuppacked light brown sugar
seeds of 1 vanilla bean (or 1 teaspoon vanilla extract)
1tablespoonbutter, in 1/4" dice
Prepare the crust:
In a large bowl, combine the flours, sugar and salt. Scatter the butter chunks over the top and work with your fingertips until the mixture looks like cornmeal, with some pea-sized butter bits remaining. Dribble 1 tablespoon of the ice water over the top, tossing the mixture with your fingers as you go, until the dough holds together when you squeeze it. Add more water if necessary, drop by drop, until this happens.
Turn this crumbly dough out onto the counter, and fraisage it: place about 1/4 of the dough under the heel of your hand and drag it across the counter away from you, smearing the dough across the counter. Scrape the dough up with a metal bench scraper and place it back in the bowl. Repeat with the remaining dough. (Fraisage-ing the dough helps to make it extra flaky by creating thin sheets of butter within the dough, almost like puff pastry, which make it rise into many layers when it bakes.)
Gather the fraisaged dough into a ball, flatten into a 1/2" thick disc and wrap in plastic. Chill at least 1 hour, until firm, and up to 3 days.
Remove the dough from the fridge, and roll out on a lightly floured surface to an 11" circle, a scant 1/8" thick. (If it has been chilled for longer than 1 hour, let the dough soften at room temp until malleable and you can roll it without it cracking a whole lot.) Trim away the ragged edges if you like, and cut the dough into 2-3" squares. Stack the squares on a plate, cover, and chill until firm, at least 20 minutes, while you prepare the filling.
Make the filling, and assemble and bake the pandowdy:
Position a rack in the center of the oven and preheat to 375ºF.
Combine the sliced apples and rhubarb in a very large bowl. Place the brown sugar on top of the fruit, then rub the vanilla seeds into the sugar to distribute and un-clump them a bit. Sprinkle the flour, salt and lemon juice over the top, and toss everything together to combine well.
Dump the fruit mixture into a 10" oven-proof skillet (or other baking vessel - see headnote). Dot with the diced butter. Lay the dough squares over the top, covering as much of the surface as you can but leaving some small windows for steam to escape; it's fine if they overlap a bit. Brush the dough with the milk or cream, and sprinkle with the remaining tablespoon of sugar.
Bake the pandowdy until the crust is golden and the filling bubbles thickly. Let cool at least 15 minutes to thicken the juices a bit. Serve warm.
The pandowdy is best the day it is baked, but can be kept for up to 4 days in the fridge and reheated in a 350º oven for 15 minutes or so until warm to crisp up the crust.
This dessert takes inspiration from three sources: Martha Stewart's apple cranberry pandowdy, Baking Illustrated's apple pandowdy, and Deborah Madison's apple rhubarb pandowdy. The crust is adapted from Martha Stewart.I used Pink Ladies for the apples here, which have a good balance of sweet to tart, and hold their shape a bit. Any semi-firm baking apple with work, such as Fujis, Pink Pearls and Mutsus, just to name a few, or use two each Granny Smith and McIntosh.A 10" skillet gives the ideal crust-to-fruit ratio and makes for a handsome presentation, but you could probably get away with a 9" skillet or a two-quart-capacity baking dish.Serve this, as I did, with ice cream such as Honey Yogurt. Dreamy Vanilla would do, too.Nutritional values are based on one of eight servings.