In a large mixing bowl, combine the sugar, quick-cooking tapioca, salt, and nutmeg. Stir well to ensure the dry ingredients are evenly distributed.

Add the rhubarb and strawberries to the bowl. Gently toss the fruit to coat it evenly with the sugar mixture. Allow the fruit mixture to stand for about 20 minutes. This resting time helps the tapioca begin to absorb the juices from the fruit, which will help thicken the filling as it bakes.

Roll out one half of your pie crust dough to fit a 9-inch pie plate. Carefully transfer the rolled-out dough into the pie plate, pressing it gently to fit snugly against the bottom and sides.

Pour the prepared fruit filling into the pastry-lined pie plate, spreading it out evenly.

Dot the filling with butter, spacing the small pieces evenly across the top.

Roll out the second half of your pie crust dough and cut it into strips (about 1/2-inch wide) for a lattice top.

Lay half of the strips across the pie vertically, spacing them evenly.

Fold back every other strip halfway, then lay a strip horizontally across the pie. Unfold the vertical strips over the horizontal strip. Continue this process, alternating the strips until the lattice is complete.

Trim any excess dough from the edges and crimp the edges of the pie crust to seal.

Brush the lattice top with the beaten egg wash. This will give the crust a shiny, golden finish.

If desired, sprinkle coarse sugar over the top for added texture and sweetness.

Preheat your oven to 400°F (200°C).

Place the pie in the preheated oven and bake for 35 to 40 minutes, or until the filling is bubbling and the crust is golden brown and fully cooked.

If the edges of the crust start to brown too quickly, cover them with strips of aluminum foil to prevent burning.

Once baked, remove the pie from the oven and allow it to cool on a wire rack. This cooling time helps the filling set further.

Serve slices of pie warm or at room temperature with a scoop of vanilla ice cream on the side for a classic pairing.