Preheat the oven to 180°C (350°F).
Prepare a circa 24cm (9-10") round pie dish by lightly greasing it with butter, oil or non-stick spray.
Heat the oil in a frying pan over medium-high heat and add the diced bacon. Cook until it’s lightly browned with some caramelised spots, but avoid cooking it until it’s too dry and crispy. Remove the bacon from the pan, leaving the oil behind. Set aside. 1 tablespoon oil, 250 grams (9oz) smoked back bacon, diced
Lower the heat and add the finely chopped onion. Cook gently, stirring occasionally, until soft and translucent.
1 medium onion, finely chopped
Add the diced mushrooms and garlic. Cook, stirring frequently to prevent sticking and burning, until the mushrooms are slightly browned.
250 grams (9oz) mushrooms, roughly chopped, 1 teaspoon minced garlic
In a large mixing bowl, whisk together the eggs, milk, cream and Dijon mustard until fully combined.
4 large eggs, 240 millilitres (1 US cup) full cream milk, 120 millilitres (½ US cup) heavy cream, 1 teaspoon Dijon mustard
Sprinkle the self-raising flour, thyme, salt and black pepper over the egg and milk mixture.
6 tablespoons self-raising flour, ½ teaspoon dried thyme, ½ teaspoon salt, ¼ teaspoon ground black pepper
Whisk until smooth, making sure no small lumps of flour remain. Whisking at high speed for 1-2 minutes helps achieve a silky texture.
Fold the grated cheddar and Gruyere into the wet ingredients until fully combined.
100 grams (3.5oz) Gruyere cheese, grated, 100 grams (3.5oz) cheddar cheese, grated
Lastly, add the cooked bacon and mushrooms and gently fold them through until evenly distributed throughout the mixture.
Pour the quiche filling into the pie dish and use a fork to gently spread the ingredients evenly, making sure no areas are too packed or too sparse.
Optional: Add a few extra slices of mushroom on top for decoration, along with a sprinkle of thyme and black pepper (just for the looks).
Bake the quiche in the preheated oven for 30-35 minutes, or until the top is golden brown and the quiche is set. It should feel firm to the touch and not wobbly.
Allow the quiche to cool slightly and firm up before slicing and serving. Tip: For the neatest slices, let it cool completely and then lightly reheat.