Preheat oven to 350F (180C).

Parbake the crust for 20 minutes. Remove from oven and set aside.

In a large frying pan over high heat, melt butter.

Add the sliced mushrooms and saute until golden brown. Remove from heat and let cool slightly.

Sprinkle half the Swiss cheese over the pie crust. Evenly spread the mushrooms on top of the cheese, then sprinkle with the remaining cheese.

Place the crust on a baking sheet (to prevent spills).

Beat together eggs and combine with evaporated milk, dry mustard, truffle salt and pepper.

Carefully pour egg mixture (custard) over the filling ingredients.

Bake in a preheated 350F (180C) for 35 to 40 minutes or until a knife inserted near the centre comes out clean. Let stand for 5 to 10 minutes before cutting into wedges to serve.