Pound out the beef slices until they are 5mm thin. I bought already very thin beef slices that don't need pounding.
Cut the pickled cucumber into quarters and peel and chop the onions.
Sprinkle salt and pepper over the beef slices. Spread 2 large teaspoons of mustard per slice evenly. Then add 2 bacon slices, and 4 pieces of pickled cucumber and sprinkle the chopped onion all over the meat slice.
Fold the long sides of the beef slice to make sure that the filling stays inside while cooking.
Take one end of the slice and start folding up over the stuffing.
Continue rolling until its completely rolled.
Secure the end of the roll with a skewer, or kitchen twine, or use roulade clips. I use kitchen twine and I tie it around the roll as I do around a gift box.
Preheat the oven to 160 degrees.
Take a dutch oven and preheat the clarified butter on high heat.
Brown beef rolls from each side.
Pour the gravy all over the rolls. Cover the dutch oven with a lid and braise the beef rolls for at least 1,5h in the oven or until soft.