Heat 2 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 to 15 minutes. When the meat is brown, transfer it to a roasting pan.
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 slightly tender, 3-4 minutes, until the vegetables are tender. Add the beer to the other pan and reduce by half over medium heat, 305 minutes.
Cook the beef:
Add the vegetables and the reduced beer to the meat in the roasting pan containing the short ribs and add the stock plus 1 quart of water. Cover with tinfoil and cook, undisturbed, for 2½ - 3 hours or until the meat is tender.
Finish and serve:
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”, 8-10 minutes, and stir in the mustard and vinegar. Puree 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, onions and salsa verde.