Preheat your oven to 350°F (175°C). Meanwhile, heat the extra virgin olive oil in a sauté pan over medium heat until it shimmers. Add the thinly sliced yellow onions and sauté, stirring frequently, until the onions are deeply caramelized, which should take 30 to 45 minutes. Once caramelized, remove the onions from the heat and allow them to cool slightly. I always make sure to keep the heat medium-low so the onions caramelize evenly without burning.

In a medium bowl, whisk together the heavy cream, whole milk, eggs, salt, and black pepper until the mixture is smooth and well combined. Stir in the caramelized onions from Step 1 and 1 cup of the grated provolone cheese. Mix everything well so that the onions and cheese are evenly distributed throughout the filling.

Pour the prepared filling from Step 2 evenly into the prebaked 9-inch pie shell. Sprinkle the remaining 1/4 cup of grated provolone cheese over the top of the filling for a delicious golden crust as it bakes. For more flavor, I sometimes add a pinch of extra black pepper on top.

Place the filled pie shell in the preheated oven and bake until a knife inserted in the center comes out clean, about 40 to 45 minutes. If the crust starts to brown too quickly, cover the edges with foil. Remove the quiche from the oven and set it on a wire rack to cool. I let the quiche cool for at least 10 minutes before slicing it; this helps the custard set and makes for cleaner slices.