Begin by filling a large pot with water and bringing it to a boil over high heat. Add a pinch of salt to the water for extra flavor. Once boiling, reduce the heat to medium; keep it at a gentle simmer while you prepare the other ingredients.

In a large skillet, heat 1 tablespoon of olive oil over medium-high heat until shimmering. Add sliced kielbasa to the skillet in a single layer, allowing it to cook undisturbed for about 3-4 minutes until browned. Flip the kielbasa and cook for another 2-3 minutes until evenly browned and crispy. Remove from the skillet and set aside.

Lower the skillet temperature to medium, and add another drizzle of olive oil if needed. Toss in sliced bell peppers and onions, sautéing them for about 10-15 minutes until tender and lightly caramelized. Remove from heat and set aside with the kielbasa.

Carefully add the frozen pierogies to the pot of simmering water. Cook according to the package instructions, about 5-7 minutes until they float to the surface. Remove them with a slotted spoon and drain well.

Melt 1 tablespoon of butter in the same skillet over medium heat. Once melted, add the drained pierogies in a single layer. Sauté for about 3-4 minutes until they develop a golden-brown crust. Flip to brown the other side for another 2-3 minutes.

Return the cooked kielbasa and sautéed peppers and onions to the skillet with the browned pierogies. Toss everything together, ensuring the pierogies are coated. Cook for an additional 2-3 minutes to heat through.

Sprinkle shredded cheddar cheese evenly over the skillet mixture. Cover with a lid and cook for about 2-3 minutes until the cheese is melted and bubbly. Remove from heat and serve warm, topped with sour cream if desired.