Preheat a pan to medium heat. Meanwhile chop the onion into small pieces. Once the pan is hot enough, stir the bacon and inion together, until the onion starts to turn golden and the bacon cooks.
Stir in the mushrooms and let it all cook for about 2 minutes. Then add in the spinach and keep on stiring until it cooks. Then set aside.
Preheat the oven to 392°F (200°C)
Crack open the 6 eggs in a bowl, and whisk them all together. Whisk in the milk, heavy cream, salt and pepper. Then add the spinach, mushrooms, onion an bacon and mix using a spoon or a rubber spatula.
Roll out the premade dough and place it on a pie pan, making sure to arrange the edges, or cutting extra dough if necessary.
Pour the egg mixture onto the quiche crust and bake at 392°F (200°C) for about 25 minutes, or until the egg mixture isn´t wiggly anymore and the top starts to urn golden brown.