1. Season ribs with salt and pepper. Heat oil in 12-inch skillet over medium-high heat until just smoking. Add half of ribs, meaty side down, and cook until well browned, about 5 minutes. Following photo 1, turn each rib on one side and cook until well browned, about 1 minute. Repeat with remaining sides. Transfer ribs to slow-cooker insert, arranging them meaty side down, as shown in photo 2. Repeat with remaining ribs.

2. Pour off all but 1 teaspoon fat from skillet. Add butter and reduce heat to medium. When butter has melted, add onions and cook, stirring occasionally, until well browned, 25 to 30 minutes. Stir in tomato paste and cook, coating onions with tomato paste, until paste begins to brown, about 5 minutes. Stir in beer, bring to simmer, and cook, scraping browned bits from pan bottom with wooden spoon, until foaming subsides, about 5 minutes. Remove skillet from heat and stir in tapioca, bay leaves, 1 teaspoon thyme, soy sauce, and prunes. Transfer to slow-cooker insert.

3. Set slow cooker on low, cover, and cook until ribs are fork-tender, 10 to 11 hours. (Alternately, cook on high for 4 to 5 hours.) Transfer ribs to baking dish and strain liquid into bowl. Cover and refrigerate for at least 8 hours or up to 2 days.

4. When ready to serve, use spoon to skim off hardened fat from liquid. Place short ribs, meaty side down, and liquid in Dutch oven and reheat over medium heat until warmed through, about 20 minutes. Transfer ribs to serving platter. Whisk mustard and remaining teaspoon thyme into sauce and season with salt and pepper. Pour 1 cup sauce over ribs. Sprinkle with parsley and serve, passing remaining sauce separately.