Preheat your oven to 300 F Spread the rouladen meat open and place a slice of bacon in the middle of each of them, lengthwise. Divide the onion and sour pickle slices over the bacon slices and spread some mustard on top. Fold the ends of each rouladen, roll them up and hold them together with the help of toothpicks. Operating in batches, brown the rouladens on each side in a large pan with some cooking fat. Place the browned rouladens in a roasting pan, pour in the chicken stock, cover and place in the preheated oven for about 60 minutes.Remove from the oven, remove the toothpicks and serve with the liquid poured on top.