Preheat the oven: Set your oven to 375°F (190°C).

Prepare the pie crust: Roll out the pre-made pie crust into a 9-inch pie pan. Use a fork to poke small holes in the crust to prevent bubbling. Bake for 10 minutes until slightly golden, then set aside.

Sauté onions: In a small pan, sauté the diced yellow onion over medium heat for about 2-3 minutes, until softened and slightly translucent. Set aside to cool.

Whisk the egg mixture: In a large mixing bowl, whisk together the eggs, heavy cream, salt, and pepper until fully combined and smooth.

Assemble the quiche: Evenly spread the sautéed yellow onions on the pre-baked pie crust. Add the shredded cheddar, mozzarella, and Swiss cheeses on top, spreading them evenly. Pour the egg mixture over the cheese and onions.

Top with green onions: Sprinkle the finely chopped green onions over the top of the egg mixture.

Bake: Place the quiche in the oven and bake for 35-40 minutes, or until the top is golden and the center is set. To check if it’s done, insert a toothpick into the middle; it should come out clean.

Cool and serve: Let the quiche cool for 10 minutes before slicing and serving. Enjoy warm or at room temperature.