Preheat the oven to 375°F. Lightly grease a 9 inch pie dish or similar baking dish and set aside.
Heat the olive oil in a large skillet over medium heat. Add the sliced mushrooms and cook for 4 to 5 minutes until softened.
Stir in the minced garlic and cook for about 30 seconds until fragrant. Add the spinach and cook for 2 to 3 minutes, stirring often, until fully wilted. Remove from heat and let cool slightly.
In a large bowl, whisk together the eggs, milk, salt, pepper, and onion powder until well combined. Stir in the shredded cheddar cheese.
Add the cooled spinach and mushroom mixture to the egg mixture and stir to combine.
Pour the mixture into the prepared baking dish and spread it evenly.
Bake for 35 to 40 minutes, or until the center is set and the top is lightly golden. Let the quiche cool for a few minutes before slicing and serving.