Blueberry swamp pie is a messy but delicious dessert that combines a streusel-topped blueberry pie with a vanilla custard filling - the best of both worlds!
In a large bowl, whisk together the flour and sea salt. Add the butter cubes to the bowl and toss to coat in the flour, then use your fingers or a pastry cutter to cut the butter into the dough until the largest pieces are the size of peas.
Slowly pour in the water, a tbsp or two at a time, mixing the dough lightly with your hands after each addition. Add only enough water until the dough starts to come together. You may need a few more or less tbsp than the recipe states.
Once the dough comes together, form it into an even disk, about 1-inch thick. Wrap tightly in plastic wrap and refrigerate for at least 30 minutes or up to 2 days.
Once chilled, roll out your dough on a lightly-floured work surface to a 12-inch circle and then fit into a 9-inch deep dish pie plate, crimping the edges as desired. Dock the crust all over with a fork and place in the refrigerator to chill for at least another 30 minutes.
To parbake the crust, heat oven to 425F. Line the chilled crust with parchment paper and top with pie weights to fill the crust entirely. Place in the oven and bake for 15-17 minutes, or until the edges are set and just starting to brown. Remove from the oven and lift the parchment and weights out of the pie shell. Return the crust to the oven and bake for another 3-5 minutes, or until the bottom appears set. Allow to cool completely.
For the blueberry filling
Toss the blueberries with the lemon zest and juice in a medium bowl. Add in the vanilla and toss again.
In a small bowl, whisk together the sugar, flour, and salt. Sprinkle this over the blueberries and mix until well combined.
For the streusel
Combine the oats, all purpose flour, whole wheat flour, brown sugar, baking powder, salt, and cinnamon in a medium bowl. Whisk to combine. Add in the butter and use your fingers or a pastry cutter to to mix it into the dry ingredients until it forms large clumps.
For the custard
In a medium bowl, whisk together the cream, brown sugar, egg, and vanilla.
To assemble
Heat oven to 375F with a rack in the lower third of the oven.
Place the parbaked crust on a parchment-lined baking sheet. Pour in the blueberry filling in an even layer. Sprinkle the streusel over the top. Place in the oven and bake until the berries are bubbling and the crust is golden, about 35-40 minutes.
Assemble the custard and pull out the oven rack. Pour the cream mixture over the pie and allow it to be absorbed into the filling.
Place back in the oven and bake for another 10-15 minutes, or until the custard is set at the edges of the pie but still slightly jiggly in the center. Allow to cool completely before serving.