Prepare your pie crust by rolling it out to about ⅛-inch thickness and transferring it to a 9-inch pie dish. Trim edges leaving a ½-inch overhang, then fold under and crimp decoratively. Refrigerate for 15 minutes while preheating your oven to 375°F (190°C).
Blind bake the crust by lining it with parchment paper and filling with pie weights or dried beans. Bake for 15 minutes, then remove weights and parchment, brush with egg white, and bake for 5 more minutes. This prevents the dreaded “soggy bottom.”
Prepare the fruit filling by combining the chopped rhubarb and strawberries in a large bowl. Toss with ¼ cup of the sugar and the lemon juice, then let sit for 10 minutes to draw out some moisture.
Make the custard mixture by whisking together the remaining ¾ cup sugar, eggs, flour, heavy cream, vanilla, salt, and melted butter in a separate bowl until smooth and well combined.
Arrange the fruit in the par-baked pie shell, spreading it evenly. The fruit should come about ¾ of the way up the crust.
Pour the custard carefully over the fruit mixture, allowing it to seep between the fruit pieces. The custard should just barely reach the top of the fruit, not completely submerge it.
Protect the edges of your pie with aluminum foil or a pie shield to prevent over-browning. Place the pie on a baking sheet to catch any potential overflow.
Bake for 45-50 minutes at 375°F until the custard is mostly set but still has a slight wobble in the center. The filling should reach an internal temperature of about 170°F for food safety.
Cool completely on a wire rack for at least 3 hours before slicing. The custard will continue to set as it cools, giving you clean, beautiful slices.