Preheat the oven to 300°F (150°C). Pat the bone-in beef short ribs dry, then season all sides with kosher salt and black pepper.

Peel and chop the yellow onion and carrots, then chop the celery stalks into bite-size pieces.

Heat the olive oil in the Dutch oven over medium-high heat. Sear the seasoned short ribs on all sides until deeply browned, about 8 to 10 minutes total.

Transfer the browned short ribs to a plate. Add the chopped yellow onion, carrot, and celery to the Dutch oven and cook until softened, about 5 minutes.

Stir the brown sugar and tomato paste into the vegetables. Cook for 1 to 2 minutes until the tomato paste darkens slightly.

Pour the porter beer and beef broth into the Dutch oven. Scrape the bottom of the pot to loosen the browned bits.

Return the seared short ribs to the braising liquid. Add the fresh thyme and bay leaf, arranging the ribs bone-side down with liquid coming about halfway up the meat.

Cover the Dutch oven and bake at 300°F (150°C) for 180 minutes. Cook until the meat is fork-tender and nearly falls off the bone.

Remove the thyme sprigs and bay leaves. Serve the beer-braised short ribs and vegetables over creamy mashed potatoes with spoonfuls of braising sauce.