Add chopped bacon, onions and halved brussel sprouts to a cast iron skillet over medium heat. Season with salt and pepper and cook, stirring occasionally, until bacon is cooked and Brussels sprouts have begun to brown, about 15 to 20 minutes.
Drizzle 1/2 cup of the balsamic vinegar over bacon and Brussels, stir and cook a minute more.
Take off a heat. Transfer to serving platter and drizzle remaining balsamic vinegar over top. Enjoy.
Prepare the tart's bacon "crust" by taking a small cast iron pan and covering the bottom of the pan with a pinwheel of bacon. Each bacon slice should have one end at the center of the pan with about half of the slice extending over the edge of the pan. (That overhang will later be folded back to form the top layer of the "crust".) Overlap bacon slices slightly until entire pan bottom is covered.
Add layer of potato slices, then layer of cheese, then potatoes, then cheese, and so on until ingredients are used and pan is filled.
Fold the bacon slices that are hanging over the edge of the pan back over the layers of potatoes and cheese towards the pan's center to form another pinwheel as the top "crust" of the tart.