In a saucepan, melt the butter then add the flour.
Using a wooden spoon, stir briskly for 1 minute to obtain a roux. Pour slowly the milk over and continue whisking. Bring to the boil and stir continuously with a whick or a spatula for 2 -3 minutes until the mixture boils and become thick and smooth.
Remove from the heat and add some mustard, the cheddar and egg yolk stirring until fully combined. Check the seasoning.
In the meantime, in a large sauté pan, drizzle some oil and warm the croissant one at the time until toasted.
Then, add some tomato slices on the sauté pan and cook for a further 2-3 minutes.
Place the tomato slices over the toasted croissant add some ham and pour over the cheesy sauce.