Instructions

1. Preheat oven to 300 degrees.
2. Combine mustard and horseradish. Divide and spread mixture thinly over one side of each beef slice. Place one slice of bacon across each. Trim bacon if necessary so it does not extend beyond edges of beef. Sprinkle with leeks or onion. Season with freshly ground pepper. Tightly roll and secure with kitchen twine in one or two places on roll.
3. Dredge beef rolls in flour. In a heavy-bottomed sauté pan, heat oil over medium-high heat until hot but not smoking. Working in batches, brown rolls on all sides, turning with tongs as required. Place browned rolls in baking dish large enough to accommodate 8 rolls.
4. Deglaze pan with red wine and whisk to remove browned pieces from bottom and edges of pan. Reduce wine over high heat a minute then reduce heat to low. Make a sauce by whisking 3 tablespoons of the flour into the wine and pan drippings. Slowly whisk in stock taking care not to let lumps form. Simmer, whisking occasionally, until thickened and stir in garlic and herbs. Season to taste with kosher salt and freshly ground pepper.
5. Pour gravy over beef roll and cover dish. Place on middle rack of oven and bake 1 hour, or until beef is very tender. (You may also simmer on top of the stove, if desired.)
6. Remove beef from gravy and tent with foil. If gravy is too thin, pour into a medium-sized sauce pan and slowly whisk in 1 tablespoon of flour. Whisking constantly, cook over medium heat until thickened.