Preheat a grill on medium or an oven to 375 degrees.
In a pot of boiling water cook the pierogies according to the directions on the package. Drain and set aside.
In the meantime, in a saucepan on low medium heat add all of the ingredients for the saucy onions. Bring to a low simmer until the liquid reduces and becomes a thick syrup consistency. Stirring occasionally. Then set aside.
If using a grill: cook the kielbasa on both sides to get a nice char on the outside and to an internal temperature of 150 degrees using a thermometer. If using an oven place the kielbasa on a foil lined baking sheet and cook on both sides to an internal temperature of 150 degrees using a thermometer. Lightly toast the rolls.
In a nonstick pan on medium heat, melt the butter and oil together. Once the butter starts to sizzle, brown the pierogies on both sides until they are golden brown.
Place the kielbasa on the roll with sauerkraut and saucy onions. Serve with pierogies and sour cream. Garnish with scallions.