Preheat oven to 350°F. Line a baking sheet with parchment and set aside.
Make the filling: In a large bowl, add the eggs, mascarpone cheese, sugar, vanilla extract, salt, and flour. Whisk everything together until well-combined.
Fill the pie: Pour the chopped rhubarb into the par-baked pie shell. Then pour the filling mixture over the top.
Bake: Place the pie on the prepared baking sheet and bake it in the preheated 350°F oven until the custard is set around the edge with a slight jiggle in the middle, 50-60 minutes. If the crust begins to brown too much, cover with a piece of foil (or pie crust cover) until fully baked.
Cool: Remove from the oven and let cool to room temperature. Chill for 4-6 hours before cutting.
