Preheat your oven to 375°F (190°C) to ensure it's hot when you bake the quiche.
Carefully place the pre-made pie crust into a 9-inch pie dish. Using a fork, gently prick the bottom of the crust multiple times to prevent bubbling during the baking process.
In a large mixing bowl, vigorously whisk together the eggs, heavy cream, and milk until the mixture is fully blended and smooth.
Fold in the shredded cheddar cheese along with the chopped mixed vegetables, dried thyme, and season generously with salt and pepper. Use a spatula to mix everything together until all ingredients are evenly distributed.
Pour the creamy egg and vegetable mixture into the prepared pie crust. Fill it to just below the edge to allow for any rising during baking.
Place the quiche in the preheated oven and bake for 35-40 minutes. The quiche is done when the top is a golden brown and a knife inserted into the center comes out clean.
Once baked, carefully remove the quiche from the oven and allow it to cool for about 10 minutes before slicing; this will help set the filling for cleaner slices.
Before serving, sprinkle chopped fresh parsley on top for an appealing finish and a touch of freshness.