Begin by preheating your oven to a temperature of 425 degrees Fahrenheit.

Proceed to peel the onions. Once peeled, slice them crossways into halves.

In an ovenproof casserole dish, large enough to accommodate all of the onion halves, combine the olive oil and butter.

To the oil and butter mixture, whisk in the balsamic vinegar, dry white wine, a dash of salt, a sprinkle of sugar, and a pinch of pepper to taste.

Arrange the onion halves in the dish, ensuring they are placed cut side down, in a single layer.

Cover the dish loosely with aluminum foil and bake in the preheated oven for a duration of 20 minutes.

After 20 minutes, remove the dish from the oven. Turn each onion half over and brush them with the pan juices to ensure they are well-coated.

Return the dish to the oven, uncovered this time, and continue to bake for an additional 20 to 25 minutes. The onions should be tender and have a browned appearance when done.

Once baking is complete, remove the onions from the oven. They can be served either warm or at room temperature, garnished with the freshly chopped green herbs of your choice.