Preheat Oven: Preheat your oven to 350°F (175°C).
Caramelize Onions: In a large skillet, melt the butter over medium heat. Add the sliced onions and cook, stirring occasionally, until they are caramelized and golden, about 20 minutes.
Prepare Pie Crust: Spread the caramelized onions evenly into the bottom of your pre-baked pie crust.
Whisk Egg Mixture: In a medium bowl, whisk together the eggs, milk, and heavy cream until well combined. Stir in the grated cheese, salt, and pepper.
Assemble and Bake: Pour the egg mixture over the onions in the pie crust. Sprinkle paprika on top for a pop of color and a hint of smoky flavor. Place the pie on a baking sheet and bake for 40-45 minutes, or until the center is set and the top is lightly golden.
Let It Set: Allow the pie to sit for at least 10 minutes before slicing. This allows it to set and makes it easier to serve.