Heat the oven to 425°F and oil a rimmed sheet. Soak bread in milk for 5 minutes and mash into a paste, then gently mix with chicken, egg, meatball salt, 1/4 teaspoon thyme and pepper. Shape 24 meatballs and bake for 16 to 20 minutes until the centers reach 165°F.
Meanwhile, cook bacon in a wide saucepan over medium heat for 7 to 8 minutes until browned. Retain 1 tablespoon fat in the pan with the bacon, discard excess and add mushrooms and pearl onions; cook for 8 minutes until mushrooms brown.
Stir in tomato paste for 1 minute, add red wine and simmer for 5 minutes. Add broth, bay leaf and remaining thyme and return to a simmer.
Transfer hot sauce and hot meatballs to a 5-quart slow cooker. Cover and cook on HIGH for 2 hours, turning gently once. Whisk cornstarch with slurry water, fold into the sauce and cover on HIGH for 15 to 20 minutes until thickened and meatball centers remain at least 165°F.
During the final 15 minutes, boil noodles in a large pot of water seasoned with noodle-water salt for 7 to 9 minutes, or the package time, until tender. Drain and toss with butter.
Remove the bay leaf. Divide noodles among 6 bowls and add 4 meatballs, mushrooms, pearl onions, bacon and wine gravy to each serving.