Preheat the oven to 390 degrees F ( 200 C ).
Add the minced garlic and olive oil to a pan and saute for 1 minute or until soft, then add the spinach and sauté until tender.
Remove the spinach from the pan to cool down and repeat the same process, but this time with the mushrooms, sauté the mushrooms in garlic and olive oil for 3 minutes and set them aside to cool down. Ensure you remove all the water from the spinach and mushrooms.
Meanwhile, add the eggs, heavy cream, salt, and black pepper to a mixing bowl and beat to combine everything.
Then, add the mozzarella cheese, spinach, and mushrooms to the mixing bowl and mix to combine all the ingredients.
Transfer the quiche mixture into a pie dish and evenly spread the mushrooms, mozzarella, and spinach throughout the dish with the help of a fork if needed.
Bake for about 35 to 40 minutes or until golden. Serve warm or cold with a salad on the side.