Method

For the pastry:

1. Preheat the oven to 200°C / 400°F / Gas Mark 6 and grease the dish
2. To prepare the pastry put the oats, flour and salt in the bowl and mix together; then add the butter and mix until you get a pebbly course texture
3. Add the buttermilk and mix to combine
4. Form the mixture into a ball and place it between 2 large pieces of cling film
5. Roll out into a circle about 10 inches in diameter
6. Remove the top sheet of cling film
7. Transfer the pastry, still on the other piece of cling film to the pie dish; then remove the remaining piece of cling film
8. Press the pastry gently into the dish
9. Bake for 9 minutes, then let cool

For the filling:

1. To prepare the filling, heat 2 teaspoons of oil in a large nonstick pan over a low heat
2. Add the onion and cook, stirring occasionally, until golden brown and caramelized, about 15 minutes
3. Transfer the onions to a bowl
4. Add the remaining 2 teaspoons of oil to the pan and heat over a medium-high heat
5. Add the mushrooms, stirring occasionally, and cook until they begin to brown, about 6 minutes
6. Add the onions back to the pan, stir in the salt, pepper, mustard and thyme
7. In a medium bowl whisk together the eggs, egg whites and evaporated milk
8. Sprinkle the cheese into the pastry case
9. Top with the mushroom-onion mixture and pour the egg mixture on top
10. Reduce oven temperature to 180°C / 350°F / Gas Mark 4
11. Bake for 40 minutes or until knife inserted in the middle comes out clean