Pat 4 ribs dry, then season with 2 teaspoons kosher salt and 1 teaspoon black pepper. Heat 2 tablespoons avocado oil in a 12-inch cast-iron skillet over medium-high until the surface reaches about 425°F. Sear the ribs for 2 to 3 minutes per broad side. Turn with tongs and move each piece only when the crust releases. The beef should turn deep amber at the edges while the centers stay raw and dark red. You'll hear a steady hard sizzle and smell toasted beef. If the pan drops below 400°F, the ribs steam and the fond stays pale. Go past 3 minutes per side and the exterior can scorch before braising. High heat triggers fast Maillard browning and builds the sauce base.

Dice 1 large yellow onion into ½-inch pieces, cut 2 carrots into ¼-inch coins and mince 4 garlic cloves. Lower the skillet to medium, about 325°F at the surface. Leave 1 tablespoon rendered fat behind. Cook the onion and carrot for 6 minutes while stirring every 45 seconds. Add garlic and double-concentrated tomato paste, then press the paste across the skillet for 2 minutes. The onion should look translucent at the edges and the carrot should deepen from bright orange to amber-orange. The double-concentrated tomato paste turns brick red and smells sweet instead of raw. Stop if the garlic darkens beyond pale gold, because it will taste bitter. Steady medium heat softens the vegetables while concentrating the tomato sugars.

Pour in 2 cups low-sodium beef stock, 2 tablespoons Worcestershire sauce and 1½ tablespoons balsamic vinegar. Scrape the skillet firmly for 90 seconds, then transfer everything to a 6-quart slow cooker. Set the 4 ribs in one snug uneven layer with the bones angled downward. Add 6 thyme sprigs, 1 rosemary sprig and 2 bay leaves. Cook on LOW near 200°F for 7½ to 8½ hours. The Crockpot Beef Short Ribs should shrink about ½ inch from the bone. They should reach 200°F to 205°F internally. The sauce will bubble gently at the edges and smell deeply roasted. (I tested HIGH first - the sauce reduced too fast around the edges. LOW stayed silkier.) Stop below 195°F and the collagen can stay chewy. Slow heat converts collagen into gelatin without squeezing out the meat's moisture.

Lift the 4 ribs onto a warm platter, then strain 3 cups cooking liquid into a 3-quart saucepan. Skim the surface fat. Set the burner to medium, about 5 on a 10-dial. Simmer the liquid for 8 to 10 minutes. Whisk 1½ tablespoons cornstarch with 2 tablespoons cold water. Stream in the slurry while whisking, then simmer exactly 90 seconds before coating the ribs twice. The sauce should turn glossy mahogany and coat the back of a spoon in a clean even layer. You'll see slow bubbles and smell balsamic sweetness beneath the beef aroma. Boil longer than 2 minutes after adding cornstarch and the sauce can thin again. A brief starch simmer thickens the reduced gelatin without dulling its shine.

Rest the glazed ribs for 8 minutes until their surface settles near 160°F. Arrange 1 rib per serving on shallow white plates, then spoon 2 tablespoons warm sauce over each portion. Pull one rib slightly open with two forks to reveal the dark moist fibers. Finish with 1½ teaspoons chopped flat-leaf parsley per plate and serve within 5 minutes. The finished Crockpot Beef Short Ribs should look irregular and glossy, with darker edges and exposed bone ends. The meat should separate in thick strands instead of collapsing into dry shreds. Serve too late and the glaze starts forming a skin. A short rest lets gelatin thicken while the beef stays juicy.