Preheat a large pan to medium heat and add the butter. Once melted, add the diced onions. Cook for about 45 minutes, stirring every 5 minutes or so. If the bottom of the pan dries up, add a couple of tablespoons of water and mix. Continue cooking until the onions have a deep brown caramelization.

Meanwhile, add the feta, cream cheese, and sour cream to a food processor or a blender and mix on high until smooth. Place in the fridge until you're ready to assemble.

Spray the 24-cup mini muffin tin with cooking spray. Unwrap the thawed pastry sheet and cut it into 24 squares. Press the pastry square into the bottom of each muffin tin. When pressed into the muffin tin, the pointy part of the pastry should stick up on four sides. You may have some puff pastry left over, and that's okay!

Spoon a small scoop of the feta mixture into each cup and top with a scoop of the caramelized onions. I like to add a lot!

Bake for about 20-25 minutes until fully cooked and golden brown.

Let it cool in the muffin tin for a few minutes, then remove and place on a serving platter. Serve immediately and enjoy!