Step 1: Set the Short Ribs in the Slow Cooker - Place the beef short ribs in the bottom of your slow cooker in a fairly even layer. They do not need to look perfect, and a little overlap is fine, but try not to stack every piece directly on top of another. Giving the ribs some contact with the cooking liquid helps them cook evenly and soak up the seasoning. I usually take a moment here to check for any small bone fragments, since short ribs can occasionally have a loose piece left from cutting.
Step 2: Add the Onion and Seasonings - Scatter the sliced onion over and around the short ribs. Add the onion powder, dried thyme, paprika, salt, black pepper, and minced garlic. Use a large spoon or clean hands to move the seasonings around so they are not sitting in one little pile. You want those warm, savory flavors touching the beef and beginning to season the broth from the first hour of cooking.
Step 3: Add the Carrots and Braising Liquid - Tuck the large carrot pieces around the ribs, keeping them chunky so they hold their shape. Stir the tomato paste into the beef broth until it is mostly smooth, then pour that mixture into the slow cooker. Give everything a gentle stir, just enough to spread the liquid and seasonings without turning the slow cooker into a wrestling match. Put the lid on securely so the heat and moisture stay trapped inside.
Step 4: Cook Until the Beef Begins to Soften - Cook the short ribs on Low for 6 hours or on High for 4 hours. Avoid lifting the lid during the early part of cooking, since every peek lets out heat and can slow things down. About 1 hour before the total cooking time is complete, open the lid and gently stir the onions, carrots, and sauce around the ribs. By this point, the kitchen should smell deeply savory, and the beef should already be starting to relax and soften.
Step 5: Mix and Add the Cornstarch - In a small bowl, whisk the cornstarch with the cold water until the mixture is completely smooth. This is sometimes called a slurry, but all you really need to remember is that cold water prevents the cornstarch from forming stubborn lumps. Pour it into the slow cooker while stirring the hot liquid gently. Try to distribute it through the sauce rather than pouring it over one piece of meat.
Step 6: Finish the Sauce and Check the Beef - Cover the slow cooker again and continue cooking for the final hour. The sauce will gradually thicken and become glossy, while the short ribs finish turning tender. Check the meat with a fork near the end. It should pull apart easily without much pressure. Taste the sauce and adjust the salt or pepper if needed, then spoon the ribs, carrots, onions, and plenty of gravy onto plates. Finish with chopped parsley right before serving.