Preheat the oven to 325°F / 165°C. Spread 8 slices of top round beef, about ¼in / 5mm thick, with ⅓ cup of German mustard.
Divide 8 slices of bacon, 8 medium German pickles, and 1 chopped yellow onion between the slices. Roll up tightly and secure with toothpicks or kitchen twine.
Heat 1 tablespoon of butter and 1 tablespoon of oil in a Dutch oven. Brown the rolls well on all sides and set aside, leaving the browned bits in the pot.
Soften 1 medium chopped yellow onion in the pot. Add 1 minced garlic clove for 1 minute, then 1 small chopped leek, 1 large chopped carrot, and 1 large chopped celery stalk for 5 minutes.
Add 1 cup of dry red wine and boil to reduce it. Stir in 2 cups of strong beef broth, 1 tablespoon of tomato paste, 1 bay leaf, 1 teaspoon of sugar, ½ teaspoon of salt, and ¼ teaspoon of black pepper.
Return the rolls, cover, and braise in the oven for about 1½ hours, until fork-tender.
Lift out the rolls. Strain the liquid back into the pot, bring to a simmer, and stir in 2 tablespoons of cornstarch mixed with 3 tablespoons of water until thickened.
Whisk in 4 tablespoons of chilled butter and season with salt, pepper, and a little more mustard. Remove the toothpicks or twine and serve the rolls with the gravy.
