Set a 12-inch nonstick or well-seasoned cast-iron skillet with a lid over medium heat.

Add 1 tablespoon of the butter to the skillet.

When the butter melts, add the onion and 1/2 teaspoon of the salt to the skillet.

Cook the onion for 4 to 5 minutes, stirring often, until it is soft and lightly golden.

Add the cabbage to the skillet in a few handfuls so it is easier to stir.

Season the cabbage with 1/4 teaspoon of the pepper.

Cook the cabbage for 6 to 8 minutes, stirring often, until it wilts down and starts to brown at the edges.

Transfer the onion and cabbage mixture from the skillet to a bowl or plate.

Add the remaining 1 tablespoon butter to the same skillet.

Put the frozen pierogi into the skillet in a mostly even layer.

Pour 1/3 cup water into the skillet, cover with the lid, and cook for 4 minutes so the pierogi heat through.

Uncover the skillet and cook for 1 to 2 minutes, until the water evaporates.