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Method

1. Start by making the béchamel. Put the milk and thyme into a small saucepan. Gently bring to a simmer over a low heat.

2. Meanwhile, melt the butter in another small saucepan, then add the flour and a pinch of salt and pepper. Cook over a low heat for about 1 minute to get rid of the floury taste, but don't let it brown.

3. Gradually whisk half of the hot milk into the roux and stir quickly to combine. Add the remaining milk and cook for 5 minutes over a low heat, stirring continuously. Remove from the heat, add the mustards and cheese and stir until the cheese has melted.

4. Transfer the sauce to a bowl. Cut a sheet of parchment paper into a circle so that it rests on the surface of the béchamel and set aside (this will prevent the sauce from forming a skin while you assemble your croque monsieur).

5. Lightly toast one side of all the sliced bread under the grill. For each sandwich, add a generous layer of béchamel to an untoasted side of bread, then top with a layer of ham.

6. Follow by adding more béchamel, some grated ²µ°ù³Ü²âè°ù±ð and a thin layer of mustard, if using. Then top with the other slice of sourdough, toasted side up.

7. Heat a frying pan over a medium heat. When hot, add the butter then carefully add one sandwich and fry for 3–5 minutes. Gently turn the sandwich over and brown the other side for another 3–5 minutes. Continue until both sides are golden-brown and the cheese has melted. Repeat with the remaining sandwiches.