Reserve 1/4 cup of the olive oil for the marinade, and set aside.
Using the rest of the oil, pan fry the fish fillets until done. Place in a non-reactive dish, such as glass.
Wipe out the pan the fish was cooked in, and place back on the burner. Over medium heat, heat the olive oil and add in the onions. Cook until softened and light browned, about 5 minutes. Add in the garlic, and cook until fragrant, another minute.
Stir in the white wine vinegar, and bring to a boil. Remove from heat, and add in the seasonings and dried herbs. Pour this over the cooked fish, and let it come to room temperature. Cover, and refrigerate overnight.
Serve the fish at room temperature or chilled.