Pre-heat the oven to 200ºC (180º for fan assisted ovens)/400ºF/ Gas M6.
Melt half the butter and the olive oil in a medium saucepan, add the onion and crushed garlic and cook for 3-4 minutes to soften. Add the rice, stir well, pour over the stock and continue to cook the rice as per the instructions on pack. Take off the heat, remove the lid and allow it to cool. Lay a clean tea towel over the pan to remove any excess moisture.
Take a large sheet of foil, rub with the remaining butter and lay the salmon on top. Season with salt and pepper, wrap up loosely, making sure the edges are folded in to make a sealed package. Place on a baking sheet and place in the oven for 10 minutes – the salmon will only be part-cooked at this stage. Remove from the oven, open the foil and allow to cool. Increase oven temperature to 220ºC (200º for fan assisted ovens)/425ºF/ Gas M7.
In a large bowl mix the cooked rice, dill, parsley, lemon zest a generous squeeze of lemon juice and salt and pepper. Roughly chop the hard boiled eggs, add to the bowl, mix well and check for seasoning.
To assemble the pie, unroll the pastry sheets and lay one on a lined baking tray. Place half the rice mixture down the middle of the pastry leaving a border of approximately 2cm. Place the salmon on top and finally top with the remaining rice mixture. Fold the second pastry sheet in half down its length and using a sharp knife make cuts down the folded side, at a 90º angle, leaving a border of 2cm. Brush base pastry border with beaten egg, open out second pastry sheet and lay over the filling. Press the edges together and with a sharp knife knock up edges to seal well. Brush the top with beaten egg and bake for 20 -25 minutes until risen and golden brown. Serve warm or at room temperature.