Preheat your oven to 350°F and position a rack in the lower third of the oven—this helps the bottom crust cook properly without the top browning too quickly. While the oven heats, remove your frozen pie crust from the freezer and let it thaw for about 10 minutes at room temperature; it should be just flexible enough to handle but still cold. Grate your Gruyere cheese fresh if you haven't already (pre-shredded cheese contains anti-caking agents that prevent smooth melting), and gather all your ingredients nearby so you can work efficiently.

In a large mixing bowl, combine the diced ham, chopped green onions, freshly grated Gruyere cheese, and grated Parmesan cheese. Toss everything together gently to distribute the ingredients evenly throughout the mixture—this ensures every slice of quiche will have balanced flavors and texture. I like to use freshly grated Gruyere because it melts into the custard much more smoothly than pre-shredded varieties, creating a creamier final texture.

In a separate bowl, whisk together the room-temperature eggs, heavy cream, whole milk, and cooled melted butter until well combined and slightly frothy—about 1 to 2 minutes of whisking. Add the garlic powder, onion powder, and freshly ground black pepper, whisking until the spices are fully distributed and the mixture is smooth and homogeneous. The room-temperature eggs incorporate more smoothly into the cream mixture, creating a silkier custard that sets more evenly throughout the quiche.

Pour the custard mixture from Step 3 over the ham and cheese mixture from Step 2, then gently fold everything together using a spatula until the ingredients are evenly distributed and no dry pockets remain—don't overmix, as this can incorporate excess air. Transfer the entire quiche filling into the thawed frozen pie crust, spreading it evenly. The filling should reach just below the rim of the crust; if it seems overly full, you can carefully remove a spoonful to prevent spillover during baking.

Place the assembled quiche on the lower third rack of your preheated 350°F oven and bake for 50 to 60 minutes. The quiche is done when the top is lightly golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs—the center should be barely jiggly if you gently shake the pan, as carryover cooking will continue to set it. If the crust edges begin browning too quickly, you can loosely tent the top with foil for the final 15 minutes of baking.

Remove the quiche from the oven and let it rest at room temperature for 15 to 20 minutes before slicing. This resting period is crucial—it allows the custard to fully set and makes for clean, neat slices instead of a runny filling. The quiche will continue to cook slightly from residual heat, so don't skip this step even if you're eager to dig in!