Preheat oven to 180°C.

In a pot, cook rhubarb with half the sugar until soft, then let it cool.

Add apples, remaining sugar, lemon juice, and cornstarch to the cooled rhubarb. Mix well.

Line a pie dish with one crust, add the filling, and dot with butter.

Cover with the second crust, seal edges, and cut slits on top.

Brush with beaten egg.

Bake for 45 minutes until golden brown.