I never usually advocate freezing, but you can do this successfully with this recipe, and therefore make a few at a time and then they’re ready whenever the urge takes you.
Ingredients
20 medium-thick slices of pain de mie or milk bread (this can be day-old bread)
20 medium-thick slices of cooked ham, from your local butcher or deli-counter
To make the béchamel, melt the butter in a heavy-based saucepan over a medium heat until foaming. Sift in the flour and cook the roux mixture over a medium-low heat, stirring constantly, for 3–4 minutes to remove the raw taste of the flour and until it turns a very pale biscuit colour and smells toasty; this is important.