Preheat your oven to 450°F (232°C). Allow 10–15 minutes for it to fully heat. You'll know it's ready when the heating element glows and the oven beeps or reaches temperature.

Wash the fresh rhubarb under cool running water and pat it completely dry. Cut it into 1-inch pieces using a sharp knife. You should have about 4 cups total.

In a 9 x 5 x 2-inch baking pan, combine the cut rhubarb, sugar, flour, lemon juice, cinnamon, and ground nutmeg. Stir everything together gently with a wooden spoon until the rhubarb pieces are evenly coated and the dry ingredients are distributed throughout. The mixture should look somewhat lumpy and wet. Checkpoint: The filling is ready when all the flour is moistened and the rhubarb releases some of its pink juice.

Unroll your pie crust (thawed if frozen, about 5–10 minutes at room temperature) and carefully lay it directly on top of the filling. If the crust tears, pinch it back together—small tears won't affect the final result.

Take any overhanging crust that drapes over the edge of the pan and fold it underneath itself, then press it firmly against the rim of the dish to create a double-thick sealed edge. This prevents the crust from shrinking into the pan and helps it brown evenly. Checkpoint: The crimped edge should feel secure and sit snugly against the pan's rim.

Using the tines of a fork, prick the entire surface of the crust 15–20 times, spacing the holes about 1 inch apart. Press firmly enough to pierce through the crust but not so hard that you dent the pan. These holes allow steam to escape and prevent the crust from puffing up into a dome. Checkpoint: You should see small holes all over the crust surface.

Place the pan on the center rack of your preheated 450°F oven. Bake for exactly 10 minutes. The crust will begin to set and turn light golden. You may notice the filling starting to bubble slightly at the edges.

After 10 minutes, carefully remove the pan from the oven (use oven mitts—the pan will be hot). Reduce the oven temperature to 350°F (176°C). Return the pan to the center rack and continue baking for 25–30 minutes more. The crust should gradually turn a deeper golden brown, and the filling will bubble actively around the edges.

The pie is done when the crust is a delicate, uniform golden brown (not dark or burned) and you can see the fruit filling bubbling up through the fork holes. The rhubarb inside should be very soft—if you gently press the crust with a fork, there should be no resistance from the fruit below. Checkpoint: The top should be golden brown and the filling should bubble visibly at the edges.

Remove the pie from the oven and place it on a wire cooling rack or a heat-safe surface. Let it rest at room temperature for at least 30–45 minutes before cutting. This resting time allows the filling to set slightly and makes it easier to serve without the filling running all over the plate. The pie will continue to firm up as it cools.

After resting, slice the pie into 6 equal wedges using a sharp knife. Wipe the knife with a damp towel between cuts for clean slices. Serve warm or at room temperature.