Rinse and dry the beef leg slices. Using a sharp knife, make several deep cuts through the skin and tendons (ask your butcher to do this if preferred). Season all sides with salt and pepper, then coat completely in flour.
Heat the clarified butter in a large pan over medium-high heat until very hot. Brown the beef slices one at a time, 3-4 minutes per side. Transfer all browned beef to a large casserole or braising pot.
Reduce heat to medium. In the same pan, cook the diced smoked pork belly for 3 minutes until the fat renders. Add the diced onions, carrots, celeriac, leek, and garlic. Cook for 5-7 minutes, stirring occasionally, until the vegetables soften and begin to brown. Add the sugar beet syrup and cook for 2 minutes until caramelized. Stir in the tomato paste and cook for 1 minute. Scrape up the browned bits with the port wine to deglaze.
Pour the entire pan contents over the beef in the casserole. Add the red wine and beef or vegetable stock—the liquid should cover the meat if possible. Bring to a gentle simmer over medium heat.
Reduce heat to low and simmer gently for 2 to 2.5 hours, until the meat is very tender and easily separates from the bone. Season to taste with salt and pepper.
Carefully remove the cooked meat and keep it warm in an 80°C (176°F) oven.
Using a slotted spoon, remove about half of the braised vegetables and keep them warm in a bowl.
Puree the remaining braising liquid and vegetables with an immersion blender (or transfer to a blender and puree), then pass through a fine sieve.
Whisk in a few cold butter flakes to create a silky finished sauce.
Serve the meat with the sauce and reserved vegetables either combined in the casserole or arranged separately on plates.
Optional: Prepare this dish a day in advance by stopping after 2 hours of simmering and letting it cool to room temperature. Refrigerate overnight, then reheat and complete the cooking the next day.
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