Preheat the oven to 375°F.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sliced mushrooms, cooking until the onions are translucent and the mushrooms are tender, about 5-7 minutes.
Stir in the chopped spinach and cook for an additional 2 minutes until wilted. Remove from heat and let cool slightly.
In a mixing bowl, whisk together the eggs, whole milk, garlic powder, salt, and black pepper until well combined.
Add the cooled mushroom and spinach mixture to the egg mixture, followed by the shredded cheddar cheese. Stir until everything is evenly mixed.
Pour the mixture into a greased 9-inch pie dish or quiche pan.
Bake in the preheated oven for 30-35 minutes, or until the quiche is set and the top is lightly golden.
Allow to cool for 5-10 minutes before slicing and serving.