Smoked haddock and leek pastries

2 x 320g sheets puff pastry

1 egg, beaten

1 tsp mustard seeds

1 tsp sea salt flakes

For the filling

100g butter

1 small leek, finely sliced

2 cloves garlic, minced

600ml milk

150g natural smoked haddock, diced into chunks

50g flour

1 tbsp whole grain mustard

1 tsp Dijon mustard