Melt butter in a saucepan on medium heat then whisk in the flour. Cook for 1-2 minutes then add in the milk. Whisk until smooth and bring to a boil. Simmer for 2-3 minutes, stirring occasionally until thickened.
Place croissant halves cut side down on a baking sheet. Layer on béchamel sauce, ham, and cheese.
Bake for 8-10 minutes until cheese is melted and croissants are heated through.
Notes
Store in the fridge for up to 5 days or freeze for up to a month.