Make the cheese sauce by, melting the butter in a small saucepan. Add the flour and mix to make a roux. Add milk, whisking well to get rid of any lumps. Season with salt and freshly ground black pepper and dry mustard powder to taste. Once the sauce thickens, add the grated cheese then the eggs, and beat well to combine. Remove from the heat and allow to cool slightly.
Line a large rectangular pie dish or roasting dish with baking paper. Carefully place the pastry into the lined dish, and trim away overhanging excess. Roll the edges down to form a thicker crust. Prick the base of the pastry with a fork. Line the pastry with baking paper and top with baking beans. Bake in oven for 15 minutes.
Remove pastry from oven and spread the base with the cheese sauce. Arrange pieces of salmon over the sauce, with the bacon, peas, mint leaves, and sprinkle with extra grated gruyere cheese.
Return to the oven and cook a further 30 minutes, until cheese sauce starts to bubble through. Serve warm or cold.
Notes
Wine pairing - We love serving this pie alongside Ara Single Estate Chardonnay.