Yesterday's croissants with mustard, gruyère and lemon recipe

In my house, these are just as likely to be the Sunday morning croissants begging to be used up fast for a late Sunday tea after a long lunch. They make a welcome change for the usual boiled or scrambled egg brigade and will even cheer the children up on what is usually the gloomiest night of the week. For 6 6 verging-on-stale croissants 25g butter 25g flour 200ml full-fat milk 75g gruyère cheese, grated, with a little extra for the top 1 tablespoon wholegrain mustard zest of 1 lemon salt and freshly ground black pepper Slice the croissants in half lengthways and toast the insides. Melt the butter in a saucepan, then take the pan off the heat and mix in the fl our to make a roux. Put the roux back on the heat and cook, stirring all the time, for a few minutes. Add the milk, using a whisk to smooth out the mixture if needs be, and bring to the boil, stirring constantly. When the sauce is nice and thick, add the gruyère, mustard and lemon zest, then season with salt and pepper. Divide the mixture between the bottom halves of the toasted croissants and cover with the tops. Sprinkle over the extra gruyère and place the croissants under a hot grill for a few minutes so the cheese melts. Serve immediately. • This recipe is taken from Trish's French Kitchen by Trish Deseine (Kyle Books, £14.99) with photography by Deirdre Rooney. Buy a copy for £11.99 from the Guardian bookshop