Additional buttermilk, for brushing top before baking
Additional granulated white sugar, for sprinkling top before baking
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Preheat oven to 350F (regular bake/not fan assisted) and lightly grease a 10-inch deep dish pie plate.
Prepare the rhubarb and set aside in a large bowl.
In a large bowl, whisk together the flour, baking soda, sugar and salt. Add the chunks of butter and using your fingertips, cut or rub the butter into the flour mixture, until you have an even crumb, with pea-sized pieces of butter.
In a small bowl, whisk together the egg and buttermilk and gradually add this to the flour mixture, adding the last part in small increments and adding only as much as you need until a moist (but not sticky) dough is formed. Remove dough to a lightly floured surface and knead lightly. Divide dough into two equal pieces. Roll one half into a roughly 10-inch circle and use it to line the bottom of the pie plate.
In a small bowl, combine the white sugar for the filling with the flour for the filling. Add to the bowl with the rhubarb and toss to combine. Pour the rhubarb on top of the bottom crust. Set aside.
Roll the second piece of dough for the top. Brush the edge of the bottom dough with water, then place the top piece on and press lightly around the edges to seal. Brush the top with additional buttermilk (or milk), then sprinkle with white sugar. Cut several slits in the top to allow the steam to escape (make sure your slits are deep enough to reach the fruit layer).
Place pie plate onto a baking sheet (to catch bubble-overs) and bake in preheat oven for 45-55 minutes, or until the crust is golden browned and fruit is tender and bubbling. (I generally bake about 50 minutes with fresh rhubarb.) *If top is at risk of over-browning, loosely cover with a sheet of aluminum foil for the last part of baking. I suggest checking at about 40 minutes. I usually have to cover at about 45 minutes for the last 5-8 minutes of baking.
Remove from oven and allow to stand at least 15 minutes before eating, to allow the filling to set a bit. Serve warm with a scoop of vanilla ice cream, if you like.
1. You can make your own buttermilk if you like, by combining 3/4 milk with 2 tsp of lemon juice or white vinegar. Stir and let stand 10 minutes before using.2. For frozen rhubarb, measure from frozen, then thaw in a colander, allowing the excess moisture to drain off.3. The 1 cup of white sugar in the rhubarb filling is what I would consider minimum for rhubarb, so I wouldn't reduce it. It will make a very lightly sweet filling, with some of the rhubarb tartness still evident. If you prefer a sweeter rhubarb filling, add the extra 1/4 cup sugar to the rhubarb.Be sure to read the Ingredient and Baker's Notes above this Recipe Card, for more tips on making this recipe. You will also find Process Photos which you may find helpful when making this recipe.