Wash and dice your strawberries and mix them with a diced piece of rhubarb stalk. Then add your sugar, lemon juice, and corn starch. Mix them all together. Pour these into a large pot and cook these down until you have a nice syrupy mixture of strawberries.
Pre-heat your oven to 350F
Cut the puff pastry into several medium sized portions, making sure that you have enough pastry to put some filling and enough pastry to fold the dough back over it. Put one or two tablespoons of the filling on one half of the dough. Then fold it over the filling and crimp the sides to seal the pastry together. If some filling oozes out, it's okay. Brush them with egg yolk and melted butter.
Place them in the oven, and cook for 20-30 minutes.
Enjoy!