Caramelized Onion, Bacon, and Blue Cheese Puff Pastry Tart

medium sweet onions, sliced

butter

olive oil

good-quality balsamic vinegar

sheet frozen puff pastry (half of 17.3-ounce package), thawed

sour cream

blue cheese crumbles

slices thick-cut bacon, cut into 1-inch squares, and cooked

salt and pepper