Heat olive oil in a cast iron skillet over medium-low heat. Add onion and garlic; cook and stir until slightly browned, about 5 minutes. Increase heat to medium and add parsley; cook for 1 minute.
Pour in enough white wine to cover the bottom of the skillet. Spread onion mixture to the edges of the skillet in a circle. Season both sides of the striped bass with black pepper. Place in the center of the the skillet; push onion mixture up against the sides. Cook until slightly browned and firm, about 4 minutes per side.