Shallot, bacon & parmesan stuffed shells recipe

Make the white sauce by melting the butter in a medium pan over a medium heat. Once foaming, stir in the flour to form a paste. Add the milk, a little at a time, whisking between each addition. Bring to the boil, then reduce the heat to a simmer and stir in the mustard and 45g parmesan. Season lightly, then remove from the heat. Stir in the lemon juice.