Sauté the mushrooms in a skillet over medium heat for about 5-7 minutes, stirring occasionally until they are tender and golden brown.

Prepare the béchamel sauce by melting butter in a saucepan over medium heat. Whisk in flour and cook for 1-2 minutes until bubbly, then slowly add milk while whisking continuously until thickened, about 3-5 minutes.

Combine the sautéed mushrooms with the béchamel sauce, stirring well to ensure every bite is creamy and flavorful. Season with salt and pepper to taste, letting it simmer on low for a couple of minutes.

Assemble your sandwiches by layering slices of bread with cheese, the mushroom mixture, and more cheese on top.

Grill each sandwich in a pan over medium heat for about 3-4 minutes on each side until the bread is golden and crispy, and the cheese is melted.

Serve warm, cutting each sandwich in half to reveal the melty goodness inside.