Prepare béchamel sauce by cooking 1 tbsp butter and 1 tbsp flour over low heat, then gradually adding 1 cup milk in three portions while stirring until thickened.

Slice 10 mushrooms and 1 onion while the sauce cools.

Heat olive oil in a pan, sauté mushrooms and onions over low heat, then add 1 tbsp soy sauce, 1 tbsp mirin, 1 tbsp honey, and a pinch of pepper. Stir until well combined.

Toast 4 slices of bread until golden brown.

Spread béchamel sauce or cream cheese on the toasted bread slices.

Layer sautéed mushrooms, ham, cheese, and leafy greens on the bread.

Top with another slice of bread to complete the sandwich.

Optional: Bake in an air fryer at 180°C for 7 minutes for extra crispiness.