Heat 1 Tbsp. oil in a 12-inch nonstick frying pan over medium-low heat. (If you don’t have a nonstick pan this size, you can use a regular 12-inch or 14-inch frying pan, but add more oil to help prevent sticking.) Add onion, salt, and pepper, and cook, stirring occasionally with a rubber spatula, until onion is soft and just beginning to brown, 10-12 minutes.

Increase heat to medium-high. Add kielbasa to the pan and spread them out, keeping them in direct contact with the pan as much as possible. Cook, stirring and turning kielbasa occasionally, until sausage begins to brown on both sides and is heated through, 4-5 minutes.

Add pierogi and water to the pan and cook, covered, until the water has evaporated and a fork can easily pierce through the center of a pierogi, 5-6 minutes. Remove lid and decrease the heat to medium. Add the remaining 1 Tbsp. oil and continue cooking uncovered, stirring occasionally, until pierogi are golden brown, about 8 minutes.

Spoon the pierogi, kielbasa, and onion onto plates. Scatter parsley on top and serve with a dollop of sour cream and some sauerkraut.