Preheat your oven to 180°C (350°F) and lightly grease a baking dish.

Take your thawed filo sheets and gently place them in the dish. Brush each layer with olive oil or melted butter before adding the next.

Sauté the chopped onion and minced garlic over medium heat until soft. Add mushrooms and spinach, cooking until spinach is wilted, about 3-4 minutes.

In a bowl, whisk together the eggs, milk, cream, salt, black pepper, and nutmeg until smooth.

Gently fold the sautéed vegetables and grated cheddar cheese into the egg mixture.

Pour the creamy filling into the filo-lined dish, spreading it evenly. Fold the overhanging edges of the filo inward toward the filling.

Bake for 30-35 minutes, until the top is golden brown and filling is set but slightly wobbly in the center.

Allow the quiche to rest for about 10 minutes before slicing.