Preheat a grill to medium.

Whisk the mustard and vinegar in a large bowl. Slowly whisk in 2 tablespoons olive oil until smooth.

Toss the onion and pierogies with the remaining 2 tablespoons olive oil and season with salt and pepper. Grill pierogies onions covered, until the pierogies thaw and the onion begins to soften, about 5 minutes. Turn the pierogies and onion and continue to grill, covered, until the pierogies are cooked through and the onion is tender, 4 to 6 more minutes.

Grill the kielbasa for approximately 8-10 minutes while pierogies and onions are grilling, turning when necessary.

Transfer to the baking sheet or platter.

Slice the kielbasa into pieces and add to the bowl with the mustard dressing. Roughly chop the onion and add to the bowl along with the pierogies and parsley. Season with salt and pepper and toss.

Serve