In the bottom of a mixing bowl whisk together the vinegar, 1/2 teaspoon salt, pepper and Dijon mustard. Slowly drizzle in the olive oil and whisk until emulsified. Add the garlic, capers, leftover juices and season with salt and pepper.
Set meat in a nonreactive serving dish and scatter the onions over the top. Pour dressing over the top and marinate at room temperature for an hour or longer. Garnish with parsley and tomato slices. Serve with a potato salad or cole slaw.