Defrost the dough. Spread the dough in an oven-safe Pyrex® container. Very finely chop the onion. Sauté it until it becomes transparent (over low heat so that it does not burn) and when it is ready, add the bacon. When the bacon is done, turn off the heat. In a bowl, beat the two eggs. Add the whipping cream, and beat the mixture a little more so that it mixes well. Add the bacon and onion, straining it slightly to remove excess fat. Now you have the filling. Preheat the oven and set it to just below the highest setting. Pour the filling into the dough and pour the cheese gratin over it. Place the dish in the oven and turn it down to 400 degrees. Leave it in for 50 minutes (you can poke it to see if the filling has set). Finally, broil it for a little bit so that it browns, but not too much. Cooking time may vary for each oven. The quiche is best if left to cool slightly, and is delicious the next day.