Season the short ribs: Pat ribs dry with a paper towel and season generously with salt and pepper.
Sear the ribs for deeper flavor: Heat 1 tablespoon of olive oil in a large skillet set over medium-high heat. Sear the short ribs for 3–4 minutes per side until they’re browned all over. Don't overcrowd the skillet. Work in batches if necessary. Transfer the ribs to a clean plate and set aside.
Sauté the onion and garlic: Return the skillet to medium heat. Add an additional tablespoon of olive oil to the skillet (if needed) and add the sliced onion. Cook for 3-4 minutes, or until the onions just start to soften. Then add the whole garlic cloves and cook for another minute.
Add the tomato paste: Once your onions are soft and the garlic is fragrant, add 2 tablespoons of tomato paste directly to the skillet. Combine with the onions and let it cook 30-60 seconds, stirring continuously. Cooking the tomato paste removes the "raw" tomato flavor and helps develop a richer, deeper base for your sauce.
Deglaze the skillet: Slowly pour in about 1 cup of the beef broth and the ½ cup of red wine (if using). As the liquid starts to bubble and steam, scrape up all the flavorful brown bits (called fond) from the bottom of the skillet. Simmer for about 2–3 minutes, just enough for the flavors to start melding and the alcohol in the wine to cook off slightly.
Build the sauce in the slow cooker: Pour the skillet mixture into your slow cooker. Then, stir in the remaining broth, Worcestershire sauce, thyme, rosemary, and bay leaves.
Add the ribs: Place the short ribs in the slow cooker, nestling them into the liquid. They don’t need to be fully submerged.
Slowly cook: Cover and cook on low for 7–8 hours, turning the ribs 2–3 times during cooking. Once your short ribs are fall-off-the-bone tender, use tongs or a slotted spoon to carefully transfer them to a plate or serving dish. Cover loosely with foil to keep warm while you finish the sauce.
Strain the sauce: Place a fine mesh strainer over a medium saucepan or skillet. Carefully pour the contents of the slow cooker through the strainer to remove any solids. You should be left with a flavorful smooth liquid in the pan.Or, you can skip straining, remove the bay leaves, and blend the liquid with the solids for an even more flavorful sauce. Reduce the sauce: Place the pan with the strained liquid over medium heat and bring it to a gentle simmer. Let it cook for 20–30 minutes, stirring occasionally, until the sauce has reduced and thickened to your desired consistency.
Serve: Enjoy your short ribs over mashed potatoes or creamy polenta with sauce spooned over the top. Garnish with fresh thyme or chopped parsley if desired.