Prepare the ingredients: Wipe the mushrooms dry with paper towels, then slice them into 0.5cm thick slices. Cut the bacon into small 3cm squares. Peel and finely chop the onion. Set all ingredients aside on separate plates to make the process smoother.

Cook the filling: In a pan, melt 10g butter over low heat. Add the chopped onions and sauté for 2 minutes until they soften and become translucent. Add the bacon pieces and cook on medium heat until they are slightly crispy and their fat has rendered. Then add the mushroom slices and sauté for 3-4 minutes until the mushrooms soften and release their moisture.

Make the creamy cheese sauce: Turn the heat to medium-low and pour in 100ml of milk. Stir gently to incorporate any bits from the bottom of the pan into the milk. Add the cheese slices and stir continuously with a silicone spatula until the cheese melts and combines with the milk into a smooth sauce. Season with a pinch of black pepper (about 0.5g) and sea salt (about 0.3g). Continue to cook for 2-3 minutes until the sauce thickens enough to coat the back of a spoon. Turn off the heat and set aside.

Assemble the sandwich: Spread a thin layer of mayonnaise (about 5g) on one slice of toast. Spoon the creamy mushroom and bacon mixture onto the toast, spreading it evenly. Top with the second slice of toast, and spread a bit more mayonnaise on the top slice to help it stick. If you prefer a more compact sandwich, press it gently with your hands.

Bake the sandwich: Preheat your oven to 180°C (350°F) for 5 minutes. Place the assembled sandwich in the middle of the oven and bake for 8 minutes. Once the toast turns golden and slightly crispy, and the mayonnaise has a slight caramelized fragrance, remove from the oven. Use a serrated knife to cut the sandwich diagonally and reveal the beautifully layered filling.