Set your oven to 375 degrees Fahrenheit and let it preheat while you prepare the filling. Place your pie crust into the 9-inch pie dish and let it sit at room temperature.

Heat your large skillet over medium-high heat and add the bulk Italian sausage, breaking it apart with your wooden spoon as it cooks. Cook for about 5 to 7 minutes until no pink remains and the pieces are golden brown with some crispy edges.

Push the cooked sausage to the side of the skillet and add your diced onion to the empty space. Cook the onion for about 3 to 4 minutes until it softens and becomes translucent. Add the minced garlic and stir everything together for another minute until fragrant.

Stir the sausage and onion mixture together so everything is evenly combined. Remove the skillet from the heat and let the mixture cool slightly.

In your mixing bowl, crack the four eggs and whisk them together until the whites and yolks are fully combined. Add the heavy cream and whole milk, whisking until everything is smooth. Add the salt, pepper, and freshly grated nutmeg, whisking until well blended.

Scatter the shredded cheddar cheese evenly across the bottom of your pie crust. Spread the cooled sausage and onion mixture evenly over the cheese. Pour the custard mixture slowly and carefully over everything, filling until about three-quarters of an inch below the rim of the crust.

Place the quiche on your oven's center rack and bake for 35 to 40 minutes. The quiche is done when the edges are set and puffed slightly but the center still jiggles a tiny bit when you gently shake the pan.

Let the quiche rest on the counter for about 10 minutes before slicing. Garnish with fresh parsley if desired, then serve warm or at room temperature.