Preheat the oven to 400 degrees F.
Heat the butter in a skillet over medium heat. Add the sliced onions with a big pinch of salt and pepper. Cook, stirring often, until the onions slightly soften, about 10 minutes. Reduce the heat to low and stir in the balsamic vinegar. Continue to stir and cook the onions for 5 more minutes. The onions should be soft and slightly caramely looking. Turn off the heat.
Spray a sheet pan with olive oil or nonstick spray - make sure the bottom of the pan is covered so the pierogies don’t stick and rip. Place the pierogies on and toss them or brush them well in the olive oil. You want all edges of the pierogies covered! Sprinkles with salt, pepper, garlic powder and smoked paprika. Spoon the caramelized onions from the pan (along with any butter in in the pan) all over the pierogies.
Roast for 20 to 30 minutes, flipping the pierogies once in between. Sprinkle with fresh herbs and serve immediately with the chipotle sour cream.
Note: alternatively, you can place raw sliced onions (and even peppers) on the baking sheet and toss with olive oil and balsamic. Place the pierogies on top. This eliminates the onion caramelizing step - but they won’t be as caramely.