First things first, preheat your oven to 200°C (or about 400°F). This will prepare a warm, toasty environment for your croque monsieur.

While the oven is heating, gather all your ingredients. Chop your leftover turkey or ham into bite-sized pieces. If you’re adding mushrooms, make sure they’re sautéed until they’re golden brown. This step is crucial—the flavor they add is pure gold!

In a medium saucepan, melt the butter over medium heat. Once it’s bubbling but not browned, whisk in the flour. Cook it for about a minute—this creates a roux. Slowly add the whole milk while whisking constantly. Your goal is a smooth, creamy consistency. Continue stirring until it thickens. Take it off the heat and mix in the Dijon mustard, thyme leaves, and a pinch of salt and pepper.

Now, let’s get to the bread. Lightly toast your slices until they’re golden on both sides. This step helps prevent the sandwich from getting soggy and adds an incredible crunch.

On half of your toasted bread slices, layer the turkey or ham, followed by the sautéed mushrooms. Then, sprinkle a generous amount of Stilton and Gruyère cheese. Top with another slice of bread to create a sandwich. Repeat for all slices.

Place the sandwiches on a baking tray. Pour the béchamel sauce over each sandwich, making it generous. Don’t be shy! Finally, sprinkle with extra cheese on top. Who doesn’t love a cheesy finish?

Put the tray in your preheated oven. Bake for 15-20 minutes, or until the cheese is bubbling and golden brown. Keep an eye on it, so it doesn’t burn. That heavenly aroma will fill your kitchen!

Once baked, let them cool for a moment before slicing. Serve with a spoonful of tangy cranberry relish for a delightful contrast. You can also add a side salad for some freshness.