Preheat the oven to 375°F (190°C).
Place the refrigerated pie crust into a 9-inch pie dish, pressing it evenly against the bottom and sides. Trim any excess dough.
Prick the bottom of the crust several times with a fork to prevent bubbling.
Bake the pie crust in the preheated oven for 8 minutes. Remove and set aside.
Heat olive oil in a medium skillet over medium heat.
Add the chopped onion and cook for 3-4 minutes until softened.
Add the diced cooked chicken and chopped spinach to the skillet. Cook for another 2 minutes until spinach wilts. Remove from heat.
In a large bowl, whisk together the eggs, milk, salt, black pepper, garlic powder, and dried thyme until fully combined.
Spread the chicken, onion, and spinach mixture evenly over the pre-baked pie crust.
Sprinkle the shredded cheddar cheese evenly over the chicken mixture.
Pour the egg mixture carefully over the filling in the pie crust.
Bake the quiche in the oven for 35-40 minutes, or until the center is set and the top is lightly golden.
Remove from oven and let cool for 10 minutes before slicing and serving.