Lay the steaks on a flat surface and brush with the mustard; lightly sprinkle each with salt and pepper. Lay a slice of bacon on each steak. Layer the onions evenly onto each steak and top with the pickle slice. Fold two opposing sides of the steak in towards the center about one inch or so forming a roughly rectangle shape. Roll into a tight cylinder and secure with a toothpick or two or butchers string. Repeat with the remaining three steaks.
Brown the rouladen in a thin layer of vegetable oil in a thick bottomed sauce pot that’s just big enough to hold all four in a single layer. Brown evenly on all sides. Use additional oil if needed and when done discard all the oil.
Add the wine, beef broth and bay leaf to the pot. Bring to a simmer and braise with a tight fitting lid over a low heat for one and a half hours or until tender. When done a thin knife blade should meet no resistance when poked into the meat.
Remove the rouladen from the broth. Stir together the cornstarch and water then whisk into the broth until it thickens. Adjust the seasoning to taste. Serve immediately.