Position an oven rack in the bottom of the oven and preheat to 400°F. Lightly grease a baking pan or cookie sheet. Set aside.
Press or roll out the dough as thinly as possible to fit your rectangular baking sheet. (It should be about 1/8'' thick.) Use extra flour, if necessary, to help this process and allow the dough to rest occasionally between each rolling session. Lay the dough onto the prepared sheet; let it rest 15 minutes.
Bake for 12 minutes until crispy and light golden brown. Set aside.
While the dough is resting and baking, mix together the sour cream, milk, salt, black pepper, and nutmeg in a small bowl. Set aside.
Cook the chopped bacon until crisp in a large skillet set over medium heat. Remove the bacon and allow it to drain on a paper towel-lined plate.
Drain all but 2 tablespoons of bacon fat from the pan. Place the onion into the pan and cook until soft and translucent.
Spread the sour cream mixture onto the par-baked pizza dough. Evenly arrange slices of brie on top. Cover with an even layer of the cooked onions and then sprinkle on the cooked bacon and crumbled Roquefort.
Place the tarte back into the oven and bake until golden brown, crispy, and the cheese is melted, about 13-15 minutes. Serve hot.
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