This is an easy puff pastry canape that's sweet, savory, tangy, creamy, and crispy - what more do you need? This is a four-part process, but much of it is inactive. The onions and bacon can be prepared 2-3 days ahead and stored in the fridge; the pastry cups can be prepared up to 8 hours before you plan to eat them and stored at room temperature, uncovered.
Part 1: Caramelize onions
Slice onions and heat olive oil in a small Dutch oven. Add the onions to the pot and season with salt. Fill a cup with water and keep it next to the Dutch oven as you cook. Cook over very low heat, stirring occasionally. If the onions begin to stick to the bottom of the pan or are turning brown too quickly, add a splash of water. (You might use 2-3 cups of water over the process.) Continue cooking for 1-2 hours or until you've reached your desired done-ness. When the onions are nearly done, add a knob of butter and a splash of your favorite vinegar (red wine vinegar and balsamic vinegar are my favorites).
Part 2: Cook bacon using the cold pan method
Arrange your bacon in a cold pan off the heat. Turn the heat on very low, and don't turn it up! The bacon will take a while to cook this way, but you'll achieve a perfect crispy-chewy texture. Once the bacon is done, remove to a paper towel-lined plate and chop it into small bits once it is cooled.
Part 3: Prepare the puff cups
First, roll out a sheet of puff pastry and cut it into small squares that are slightly larger than the muffin cups. I use a mini-muffin pan that has 1.5-2 inch cups, so my squares are about 2x2 inches.
You'll need to cut up some parchment paper to prepare your mini-muffin pan. First, cut small 1-inch wide strips (one for each puff you plan to make). Then, cut the same number of squares that are approximately the same size as your puff pastry squares.
Put a parchment strip in the bottom of the muffin cup. Then, tuck the puff pastry square into the cup on top of the strip. Next, add the parchment square on top of the puff pastry. Put a few baking beans into the puff cup to weigh down the parchment. (This will ensure that the puff pastry bakes into a cup shape rather than a big cube you can't fill.)
Once you've added all your puff pastry to the muffin pan, put the whole thing in the freezer for 5-10 minutes. This ensures that the butter will evaporate before melting out, which is what allows puff pastry to puff. Bake at 400 degrees F for 15 minutes or until golden brown.
Part 4: Fill and serve
Once the pastry cups are fully cooled, spoon a dollop of caramelized onions into each cup. Top with a sprinkle of bacon pieces and garnish with parsley.