Preheat the oven to 375°F (190°C).
Place the pie crust into a 9-inch pie dish and crimp the edges.
In a large skillet, cook the sausage over medium heat for 6–8 minutes, breaking it into small crumbles.
Add the diced onion and cook for another 3 minutes until softened.
Remove the sausage mixture from the heat and let it cool slightly.
Sprinkle the cheddar and mozzarella evenly over the pie crust.
Add the cooked sausage mixture on top of the cheese.
In a large mixing bowl, whisk together the eggs, heavy cream, milk, salt, pepper, garlic powder, thyme, paprika, and parsley.
Pour the egg mixture evenly over the sausage and cheese.
Bake for 40–45 minutes, or until the center is set and the top is lightly golden.
Remove the quiche from the oven and let it rest for 15 minutes before slicing. This helps the filling set properly.
Slice into 8 portions and serve warm or at room temperature.