Pat the boneless beef chuck short ribs dry with a paper towel and season with salt and black pepper
Heat a skillet over medium-high heat and add olive oil if searing the ribs
Sear the ribs for two to three minutes per side until browned then transfer to the slow cooker or Dutch oven
Add the chopped onions and minced garlic spreading them evenly over the ribs
In a bowl whisk together beef broth tomato paste Worcestershire sauce soy sauce balsamic vinegar brown sugar and thyme
Pour the mixture over the ribs ensuring they are well coated
Add the bay leaf and cover the slow cooker with a lid
Cook on low for eight to nine hours or on high for four to five hours until the ribs are tender and fall apart easily
If using an oven preheat to 325°F cover the Dutch oven and bake for three hours checking for tenderness
If the sauce is too thin remove the ribs and pour the liquid into a saucepan over medium heat
Stir in the cornstarch slurry and cook for five to seven minutes until thickened
Return the ribs to the sauce and coat well before serving
Serve hot with mashed potatoes rice or roasted vegetables