How to Make It
Preheat the oven to 350°. Brush a large shallow baking sheet with 1/2 tablespoon of the olive oil. Spread the onion wedges on the pan, drizzle with 1 1/2 tablespoons of the oil and season with salt and pepper. Cover with foil and bake for about 25 minutes, or until soft but not brown.
Heat 1 1/2 tablespoons of the olive oil in a large skillet. Add the scallions and season with salt and pepper. Cover and cook over high heat, stirring frequently, until tender but still green, about 5 minutes. Transfer to a bowl.
Heat the remaining 1 1/2 tablespoons of oil in the skillet. Add the red onions and cook over moderate heat, stirring often, until soft but not colored, about 25 minutes. Stir in the vinegar and cook until nearly evaporated, about 1 minute. Add the port and cook for 1 minute longer.
Just before serving, combine the cooked onions and scallions in the skillet and rewarm over moderate heat. Transfer to a bowl and serve.