01 - Melt butter and olive oil together in a large skillet over medium heat.
02 - Add onions and cook for 6–8 minutes, turning occasionally until golden and slightly softened.
03 - Sprinkle brown sugar over onions and cook for 2 minutes, stirring gently to coat evenly.
04 - Pour in balsamic vinegar and vegetable stock, then season with salt and black pepper.
05 - Reduce heat to low, cover, and simmer for 10–12 minutes, stirring occasionally until onions are tender and liquid reduces to a syrupy glaze.
06 - Remove lid and cook for an additional 3–5 minutes to thicken glaze and coat onions thoroughly.
07 - Transfer to a serving dish, garnish with chopped parsley if desired, and serve warm.