In a pan of salted water boil the potatoes for 5 minutes until starting to soften but still have structure. Drain and set aside.
In a frying pan sweat the shallots in a pan with a little oil until soft and just starting to caramelise.
In a mixing bowl whisk the eggs together with the cream, mustard powder and thyme, season well. Add the potatoes, shallots and cheese to the sauce and coat well.
Lightly grease a suitable ovenproof pie dish, roll out the pastry to the thickness of a pound coin sprinkling the black onion seeds on top. Line the dish with half of the pastry trimming any excess, spoon in the cheesy shallot filling and brush the egg wash on to the exposed pastry sides. Lay over the remaining pastry, you may have to bring together the trimming and roll out again if you don’t have enough. Generously egg wash the top and use a knife to poke some vents into the top.
Bake in an oven preheated to 200C/180C Fan/Gas Mark 5 for 45-50 Minutes until golden. Serve with seasonal vegetables.