Preheat the oven to 350ºF (175ºC). If you're serving immediately, this step is crucial for achieving that perfect bake.
Peel and finely slice the onions. This step can be a little tearful, but the end result is worth it. Aim for even slices so they cook uniformly.
In a large dutch oven or a high-sided skillet, heat the butter over low heat. Wait for it to melt completely before proceeding to the next step.
Once the butter is melted, add in the sliced onions and salt. Stir well to coat the onions in the buttery goodness.
Cook the onions uncovered on low heat for about 30 minutes. You're looking for them to become soft and translucent, but not browned. This slow cooking process is essential for developing their sweetness.
After 30 minutes, increase the heat and add the water, sugar, and balsamic vinegar. As this mixture bubbles gently, the liquid will reduce, and the onions will become incredibly tender. Let this cook for an additional 10 to 15 minutes.
Once the liquid has mostly evaporated and the onions are melting tender, transfer them to a casserole dish. If you're preparing ahead of time, you can refrigerate them at this stage for up to 24 hours.
In a non-stick skillet, heat the olive oil over medium heat. Add the panko breadcrumbs, stirring continuously until they turn golden brown. This step is crucial to ensure that they don't burn.
Once toasted, remove the skillet from heat and transfer the breadcrumbs to a cold bowl immediately to halt cooking.
Sprinkle the golden breadcrumbs over the hot onions in the casserole dish. Bake for 10 minutes to meld the flavors.
Finally, sprinkle the fresh chives and parsley over the top before serving. This step adds a pop of color and freshness to your dish.
