Heat your Stargazer Cast Iron skillet to medium high heat. Add butter and let it melt.
Add the sliced onions, mix well with the butter, and bring the heat to low temperature. Let the onions cook slowly until they are brown and soft, about 25 minutes. Remove the skillet from the heat, and remove the onions and set aside, allowing them to cool (do not clean the skillet).
Preheat your oven to 300°F.
Once your skillet has cooled, spread the dough over the skillet. If the dough is square, cut the triangles that form outside of the skillet.
In a large bowl, beat the eggs. Add the heavy cream, brie cheese chunks, caramelized onions, cheddar cheese and parsley (save a bit of parsley for garnish).
Pour the eggs and cream mix over the dough. Using a pastry brush, brush the edges of the dough with a bit of the eggs and cream mixture.
Place your skillet in the oven, baking for 45 minutes or until brown on top.
Let the quiche cool and add remaining parsley for garnish. Season to taste with salt and pepper, if desired. Serve and enjoy!