Lay each beef slice flat on a cutting board and season lightly with salt and pepper. Spread about 1 teaspoon (or to taste) of mustard evenly over each piece.

Place a half slice of bacon on each piece, then add a few rings of sliced onion and 2–3 pickle slices. If using breadcrumbs or horseradish, add a small spoonful now.

Tightly roll each piece from the short end and secure with toothpicks or kitchen twine. Make sure they’re snug so the filling doesn’t fall out.

Heat oil in a large ovenproof skillet or Dutch oven over medium-high heat. Brown the roulades on all sides — about 3–4 minutes per side — until well-colored. (Tip: don’t crowd the pan; brown in batches if needed.)

Remove roulades and set aside. In the same pan, add butter and the remaining sliced onion; cook over medium heat for 4–6 minutes until soft and lightly caramelized.

Pour in red wine (or extra broth) to deglaze the pan, scraping up browned bits for extra flavor. Let the wine reduce for 2–3 minutes. Add beef broth, bay leaf, and thyme.

Nestle the roulades back into the sauce. Bring the liquid to a gentle simmer.

Cover the pan with a tight-fitting lid and transfer to a preheated oven at 325°F (160°C). Braise for 1½ to 2 hours, until the meat is tender and easily pierced with a fork. (Alternatively, simmer gently on the stovetop for the same time.)

Remove roulades to a plate and keep warm. Strain the sauce or leave the onions if you like texture. To thicken, whisk 1 tablespoon flour with a bit of cold water (or use cornstarch slurry: 1 tbsp cornstarch + 1 tbsp cold water) and simmer for 2–3 minutes until sauce coats the back of a spoon. Taste and adjust seasoning.

Remove toothpicks/twine, slice roulades if desired, spoon sauce over them, and garnish with chopped parsley. Serve hot with mashed potatoes, spaetzle, or your favorite sides.