Preheat the oven to 350ºF (175ºC).
Place the frozen deep-dish pie crust on a baking sheet.
Evenly spread the cooked chopped chicken, thawed and well-drained chopped spinach, and French fried onions in the bottom of the pie crust, then sprinkle the shredded Monterey Jack cheese on top.
In a mixing bowl, whisk together the eggs, heavy cream (or milk), sour cream, garlic powder, salt, and black pepper until smooth.
Pour the egg mixture over the chicken and cheese in the pie crust, making sure it distributes evenly.
Bake the quiche in the preheated oven for about 60 minutes, or until the center is set and a knife inserted near the center comes out mostly clean.
Let the quiche rest 5–10 minutes before slicing and serving.