1) Trim pot roast of all fat and gristle; Julieene pieces should be 1/4 x 2" long. 2) In a small mixing bowl, combine the vinegar, salt, pepper, and mustard. Whisk in the oil until mixture is emulsified or well blended. It will separate after standing. Add the capers, garlic, and half the parsley. 3) Pour dressing over meat, mix carefully. Cover and marinate 3 hours or more. Serve on shallow platter garnished with tomatoes, gherkins, sliced eggs, onions, and remaining parsley.