Add 50g of butter to a frying pan over medium heat. Add the mushrooms and sliced leeks and cook until softened. Stir through the chopped thyme and season with the Black Truffle Salt (or season to taste). Set aside.
Make the béchamel sauce by melting the butter over medium heat in a medium sized pot. Add the plain flour and whisk for about 3-4 minutes until it has cooked off. Add the mustard and mix. Slowly add the milk and whisk thoroughly to break up any lumps. Keep whisking until béchamel sauce issmooth and creamy in texture. Whisk in the Black Truffle Salt to season.
Preheat oven to Med/High (180 degrees) Grill setting.
Line a large baking sheet with baking paper. Lay out the 8 slices of bread on the baking sheet. Spread each slice with a generous layer of béchamel sauce. Top 4 of the slices with the mushroom filling. Next add grated cheese over all 8 slices. Cover the mushroom slices with the bechamel/cheese slices facing upward.
Grill in the oven for 15-20 mins or until béchamel is bubbly and golden. Season with extra Black Truffle Salt if desired.
