These beef roll-ups are pure comfort food in Germany! There are many different variations as every family will make a slightly different version. In our family we eat them with potato dumplings and red cabbage.
Spread the beef slices out on a large cutting board.
Brush each slice with 1 teaspoon mustard and season with salt and pepper.
Top each slice with 2 slices of bacon or pork belly.
Peel and quarter the onion. Cut one quarter into four thick slices and place a slice of onion on each piece of meat. Top it with a gherkin. Finely chop the rest of the onion and distribute it over the meat pieces.
Tightly roll the pieces up from the shortest side; secure the roll-ups with wooden toothpicks.
Heat oil in a wide pan or pot; add the roll-ups and brown on all sides.
Add enough stock to cover the roll-ups. Stir in 2 tablespoons mustard, chopped tomato, onion and red pepper. Bring to the boil; reduce heat to low and cook, covered, for 90 minutes. Turn the roll-ups over every so often. At the end of the cooking time, remove them from the pot and keep warm.
Pour the sauce through a fine sieve into a saucepan. Discard the solids and bring sauce to the boil. Dilute 1 to 2 tablespoons cornflour in a few tablespoons cold water and whisk it into the sauce. Cook, whisking, until the sauce thickens. Season to taste with salt and pepper, adding more mustard or tomato puree if needed.
When the sauce is to your liking, return the roll-ups to the pan and heat through before serving.