Preheat the oven to 180℃.
Grease a 9 inch pie or tart dish with some butter, both the bottom and the sides.
Heat oil and butter in a wide pan over medium heat.
Saute the garlic till it becomes pinkish in colour.
Add the sliced mushrooms and salt.
Mix well. Then increase the heat to high and allow the mushroom slices to cook without stirring them too often.
Stir only once or twice.
When they turn a little brownish and reduced in volume, add the spinach.
Give it a mix till the spinach wilts. You don't want to over cook the spinach.
Allow the mixture to cool down.
In the meantime prepare the savoury custard.
Add eggs, milk, cream in a mixing bowl.
Whisk the mixture to mix well.
Add the pepper, Dijon mustard, nutmeg, red chilli flakes and whisk a bit more.
Add the grated cheese. Mix well using a spatula. I don't whisk it as I don't want the cheese to break down.
Spread the mushroom spinach mixture over the pie or tart dish in an even layer.
Pour the custard mixture over it.
Sprinkle the chopped spring onion over the mixture.
Bake the quiche for 35 minutes or until it is done.
Insert a fork or toothpick in the middle. It should come out clean.
Allow the quiche to cool down a bit before you slice it. The middle may still be a bit jiggly.
For even firm slices allow the quiche to set for at least 2 hours. Can easily warm it up before serving.
For warm quiche can slice after 15-20 minutes but you may not get firm slices.