Preheat the oven to 400 degrees. Spray a mini muffin tin with cooking spray.
Heat a large pan on medium heat. Melt the butter in the pan. Add in the onions, ½ teaspoon of salt, and cook, stirring occasionally, until deeply caramelized (about 30 minutes). As the onions cook you will need to stir them more frequently and reduce the heat to medium-low to prevent burning. During the last minute or so of cooking, stir in the fresh thyme and a pinch of freshly cracked black pepper. Take off the heat and roughly chop the onions. Set aside.
Roll out the thawed puff pastry (see notes for tips on working with puff pastry). Slice it into approximately 2in x 2in squares. Place each square into a mini muffin cup. Fill each puff pastry with a spoonful of caramelized onions. Top each one with a dollop of goat cheese.
Bake for 25-30 minutes, or until the puff pastry is puffed and golden brown.
Serve warm with a drizzle of balsamic glaze on top.
Notes
Makes about 20 puffs.
Puff pastry tips:
Defrost your puff pastry in the refrigerator or on the counter prior to using. If you have not defrosted it enough, it will be impossible to roll out and will break when working with it.
That said, if it is too warm it can become sticky. If it is sticking, pop it back in the fridge or freezer for a few minutes before continuing to work with it.