Melt the butter in a large skillet over medium heat. Add the finely chopped onion and cook for about 4 minutes, stirring occasionally, until the onion becomes soft and translucent. This gentle cooking mellows the onion's sharp bite and creates a sweeter, more complex flavor foundation. Stir in the minced garlic and cook for another 30 seconds until fragrant, then remove from heat.

Transfer the sautéed onion mixture from Step 1 into your slow cooker. Add the frozen pre-cooked meatballs directly—no browning needed since they're already cooked. Pour in the beef broth and onion soup, then add the Worcestershire sauce, bay leaf, and dried thyme. Season with salt and black pepper to taste. Stir everything together gently to distribute the ingredients evenly.

Cover the slow cooker and cook on low for 8 hours or on high for 4 hours, depending on your schedule. The long, gentle cooking allows the flavors to meld beautifully and keeps the meatballs tender. I find that cooking on low overnight gives the deepest, most developed flavor—the extended time lets the herbs and aromatics fully infuse into the broth.

Remove the bay leaf from the slow cooker. In a small bowl, whisk together the cornstarch with 2 tablespoons of cold water until smooth and lump-free—this prevents the cornstarch from clumping when added to the hot liquid. Stir this slurry into the slow cooker, then add the heavy cream and stir well to combine. Cover and cook on high for 15 minutes to allow the cornstarch to thicken the gravy and create a silky, rich sauce.