Preheat your oven to 350°F. While the oven heats, remove the pie shell from the freezer and place it in a 9-inch pie plate. Let it sit at room temperature for just a minute or two so it's easier to work with, but keep it cold—this helps maintain that flaky texture you're after.

In a medium bowl, whisk together the sugar, 2 tablespoons flour, egg, and vanilla until well combined—this creates a custard-like base that will help thicken the rhubarb filling as it bakes. Fold in the cut rhubarb pieces gently, making sure everything is evenly coated. I like to let this sit for just a minute so the sugar starts to draw out the rhubarb's natural juices, which deepens the flavor.

In a separate bowl, combine the 3/4 cup flour, packed brown sugar, oats, and cinnamon. Cut the cold, cubed butter into the dry mixture using a pastry cutter, two knives, or your fingertips, working until the mixture resembles coarse breadcrumbs with pea-sized clumps of butter still visible. I find using cold butter straight from the fridge is the secret to getting that crumbly, crispy texture—the butter pieces create little pockets of crispness as they bake.

Pour the rhubarb filling from Step 2 into the prepared pie shell, spreading it evenly. Distribute the crisp topping from Step 3 over the filling in an even layer, letting some of it mound slightly in the center—don't pack it down, as you want it to stay crispy. Bake at 350°F for about 1 hour, until the filling bubbles slightly at the edges and the topping is golden brown.

Remove the pie from the oven and let it cool for at least 20-30 minutes before slicing. This resting time allows the filling to set properly so each slice holds together beautifully. Serve warm or at room temperature with vanilla ice cream or whipped cream if desired.