Preheat your oven to 425°F (220°C).
In a large bowl, mix together the chopped rhubarb, sugar, flour, cinnamon, and lemon juice until the rhubarb is well coated.
Lay the pre-made pie crust into a 9-inch pie dish, ensuring it fits snugly.
Pour the prepared rhubarb filling into the crust, distributing it evenly.
Scatter the diced butter over the top of the filling.
If using a second pie crust, roll it out and place it over the filling, or use the top crust from your pre-made package.
Trim the edges and crimp them together to seal the pie.
Cut several small slits in the top crust to allow steam to escape.
Brush the surface with the beaten egg and sprinkle the coarse sugar over it.
Bake the pie for 15 minutes, then lower the temperature to 350°F (175°C) and continue baking for another 30-35 minutes.
Once done, let the pie cool for at least 30 minutes before slicing.