In a large skillet, heat the olive oil over medium heat
Add the kielbasa and cook on each side until it begins to brown
Remove the browned kielbasa from the skillet and set aside
Leave the oil in the skillet
Add the bell pepper and onions to the skillet and cook over medium heat until softened, about 10-15 minutes
When the peppers and onions are cooked remove them from the skillet and set aside
Melt the butter in the same skillet over medium heat add the frozen pierogi and 1/2 cup of water and cover to cook for 6-8 minutes, stir occasionally
When the pierogi are soft and cooked, take the lid off and let them crisp up a bit
Add the sausage, peppers, onions to the skillet, toss combine, allow to reheat
Sprinkle the shredded cheddar over the pierogi and kielbasa in the skillet. Cover with a lid and allow the cheese to melt
Serve with sour cream and a dash of salt and pepper if desired