1.  Preheat the oven to 350°F.

2.  First brown the meat: Heat two skillets large enough to hold the meat in a single layer. Add half of the oil and heat until it smokes lightly. Season the short ribs liberally on both sides with salt and pepper. Place the meat in the oil and brown on all sides, 12-15 minutes. When the meat is brown, transfer it to a roasting pan.

3. Next, build the cooking liquid: In the pans where the meat was browned, add the thyme, onions, carrots, celery, and garlic to one and cook until the vegetables are tender, 3-4 minutes. Add the beer to the other pan and reduce by half over medium heat, 3-5 minutes.

4. Add the vegetables and the reduced beer to the short ribs in the roasting pan then add the stock. Cover with foil and cook, undisturbed, for 2 1/2 – 3 hours or until the meat is tender.

5. When the meat is tender, remove the pan from the oven and transfer the meat to a cooling rack. Strain the vegetables from the cooking liquid. Reserve them. Reduce the liquid on the stove until “nappe”, which means it's thick enough to coat the back of a spoon, this will take 8-10 minutes. Then stir in the mustard and vinegar. Purée some of the cooked vegetables in the blender until smooth and add it to the sauce to thicken it. Season the meat on the cooling rack. Arrange the meat on a serving platter (or on individual plates) top with sauce.