Шаг 1
Melt the butter in a skillet over medium heat. Add the finely chopped onion and cook, stirring, for 4–5 minutes until soft and translucent. Add chopped mushrooms and cook for about 5 more minutes until the mixture is cooked through and slightly browned. Season with salt and pepper. Cook another 2–3 minutes and remove from heat.
Шаг 2
Preheat the oven to 200°C (390°F).
Toast the bread in a toaster or oven. Spread mustard on one side of each slice. Place the slices mustard-side-up on a baking sheet, top with 2 slices of cooked turkey, 1/4 of the mushroom-onion mix, and sprinkle with cheese. Bake for 5–8 minutes or until the cheese is melted.