Heat 1 tbsp (15ml) of olive oil in a large skillet over medium-high heat. Add the thickly sliced kielbasa in an even layer. Let the sausage sear undisturbed for 2-3 minutes per side until you achieve a deep, smoky sear and slightly charred edges.

Remove the seared kielbasa and set it aside, leaving the flavorful drippings in the skillet. Reduce the heat to medium-low and melt 2 tbsp (30g) of butter into the pan. Add the sliced onions and cook slowly for 15-20 minutes, stirring occasionally until translucent and deeply golden-brown.

Pour the 1/2 cup (120ml) of chicken broth into the bottom of your slow cooker insert. Layer the frozen plump potato pierogies evenly at the base, nesting them together. Distribute the charred kielbasa generously throughout, then mix in the golden-brown caramelized onions.

Cover the slow cooker with its lid. Cook on LOW for 3 to 4 hours, or on HIGH for 1.5 to 2 hours. The pierogies are done when they are plump, soft to the touch, and heated completely through.

Once the cooking time is complete, remove the lid. Smother the entire dish in a thick, even layer of the gooey orange cheddar and stringy white mozzarella cheese. Place the lid back on for 5-10 minutes until the cheese is beautifully bubbling and glistening.

Turn off the slow cooker. Sprinkle the freshly chopped green parsley flakes evenly over the top as a vibrant garnish. Serve immediately hot from the crockpot.