Begin by thickly slicing the onions, julienne style. Place the onions in a pot with the butter, and cook over medium heat until caramelized. About 1 hour.

Once the onions are on the stove dice the bacon and render out all the fat, setting both aside. The fat can be used to brush the tart pan later before lining it with the dough.

Brush a 9" x 13" cookie sheet pan with butter or the rendered bacon fat, to prevent the dough from sticking to it. Roll out the chilled pie dough and line the pan with it, making sure the dough comes up the sides of the pan. Using a sharp knife trim the edges to create even straight lines/edges.

Spread the caramelized onions and rendered bacon evenly over the dough lined sheet pan.

Mix the eggs, buttermilk, cream, and half of the caraway and chopped chives in a mixing bowl with a whisk. Pour this mixture over top of the partially filled tart. Sprinkle with the rest of the chives and caraway.