This isn't an ordinary strawberry rhubarb pie. This crumb-topped pie is baked in a cast-iron skillet

Ingredients

Pie Dough

• 2 cups all-purpose flour
• 1/2 teaspoon fine sea salt
• 12 tablespoons cold unsalted butter, cut into 1/2-inch cubes
• 1/3 cup ice water, plus more as needed

Filling and Finishing

• 4 cups thickly sliced rhubarb
• 3 cups chopped strawberries
• 1 1/3 cups granulated sugar, divided
• 1/3 cup cornstarch
• 1/4 teaspoon freshly grated nutmeg
• Fine sea salt
• 1/2 teaspoon vanilla bean paste
• 1/2 cup old-fashioned oats
• 1/2 cup all-purpose flour
• 1/3 cup light or dark brown sugar
• 1/4 teaspoon baking powder
• 1/4 teaspoon ground cinnamon
• 6 tablespoons cold unsalted butter, cut into 1/4-inch cubes
• Egg wash (1 egg beaten with 1 tablespoon water; optional)
• Turbinado sugar, for sprinkling (optional)

Directions

1. In a medium bowl, whisk together the flour and salt. Using your hands, toss the butter through the flour until each piece is well coated. Cut the butter into the flour by pressing the butter between your fingers and thumbs, flattening the cubes into big shards. Continue to mix until the butter is almost completely incorporated — the mixture should resemble cornmeal in color and texture. 
2. Make a well in the center of the bowl and add the ice water. Toss the flour mixture gently (rather than stirring) to moisten and incorporate the water without overworking the flour. Continue adding water, 1 to 2 tablespoons at a time, and tossing and kneading slightly until the dough comes together. (It should hold together easily without feeling wet or sticky.)
3. Form the dough into a disk and wrap tightly in plastic wrap. Refrigerate for at least 30 minutes and up to overnight.
4. Make the strawberry-rhubarb filling: In a large pot, toss the sliced rhubarb, strawberries, and 1/3 cup of the granulated sugar to combine. Cook over medium, tossing occasionally, until the rhubarb starts to soften slightly, 5 to 7 minutes.
5. In a small bowl, whisk the remaining 2/3 cup granulated sugar, cornstarch, nutmeg, and a pinch of fine sea salt to combine. Sprinkle the sugar mixture evenly over the strawberry-rhubarb mixture in the pot and mix well to combine. Cook, stirring frequently, until the mixture gently bubbles from the center of the pot. Let cool completely before preparing the pie (to do this quickly, pour into a casserole dish or baking sheet and spread into an even layer). 
6. Make the streusel: In a medium bowl, stir the oats, all-purpose flour, whole-wheat flour, brown sugar, baking powder, cinnamon, and ¼ teaspoon fine sea salt to combine. Rub the butter into the flour mixture with your fingers until the mixture forms large clumps.
7. Heat the oven to 400°F with the oven rack placed in the lower third of the oven (and preferably with a pizza stone on the rack, though this is optional). Grab a 9-inch skillet. 
8. Assemble the pie: Roll out the chilled dough on a lightly floured surface until it's about 1/8 inch thick. Press firmly and evenly, rotating the dough as you work to prevent sticking without adding too much flour. (Ideally the dough will also be about 1 1/2 inches wider than the top edge of your skillet all the way around.) 
9. Transfer the dough to the skillet. Starting at one end of dough, wrap it around the rolling pin. Lift the pin to the edge of the skillet and unfurl the dough into it. Press the dough into the base of the skillet and trim the excess dough to 1/2 inch around the outside edge of the skillet. 
10. Pour the cooled filling into the prepared crust and sprinkle the streusel evenly on top. Fold the excess crust over the filling all around the edge of the crust, letting it pleat naturally the way you would for a galette. If desired, brush the exposed crust with egg wash and garnish with turbinado sugar.
11. Transfer the pie to the oven. Bake for 20 minutes, then lower the temperature to 350°F and continue to bake until the crust is browned and the filling is visibly bubbly, 40 to 50 minutes. 
12. Let cool completely before slicing and serving.