Start by trimming any excess fat from the beef short ribs. This will help reduce greasiness while cooking. Season them generously with salt and pepper. Heat a large, heavy pot or Dutch oven over medium-high heat. Once hot, sear the ribs, browning them on all sides. This should take about 4 minutes per side. Set the seared ribs aside on a plate.
In the same pot, add chopped carrots, celery, and walnuts. Sauté them for about 5-7 minutes until they start to soften and develop some color. Scrape the browned bits from the bottom of the pot with a wooden spoon. Those bits pack a lot of flavor.
Once the vegetables have cooked down, stir in the fresh thyme and Worcestershire sauce. The fragrance is delightful! Cook for another minute before adding the caramelized onion jam.
Pour in the smooth ale, making sure to scrape any remaining pieces stuck to the pot. Bring the mixture to a gentle simmer. Return the ribs to the pot, ensuring they are submerged in the liquid. Cover the pot with a lid and reduce the heat to low.
Allow the ribs to cook gently for at least 2.5 to 3 hours. They're done when the meat is tender and nearly falling off the bone. Your kitchen will start to fill with magical, mouthwatering aromas during this time.
While the ribs are cooking, boil the large baking potatoes in salted water until tender, about 15-20 minutes. Once cooked, drain them, mash with butter, and season with salt.
Once the beef is ready, take it off the heat. Ladle the cooked vegetables and sauce over the ribs on a platter. Serve alongside creamy horseradish and mashed potatoes.