Heat oven to 425°F; oil a rimmed sheet. Soak crumbs in milk 3 minutes. Gently mix with beef, pork, egg, grated onion, meatball salt, and pepper. Shape 24 meatballs; bake 16 to 20 minutes until centers reach 160°F.
Meanwhile, melt 2 tablespoons butter in a wide saucepan over medium-high heat. Cook mushrooms and sliced onion 10 to 12 minutes until mushroom liquid evaporates and onion starts browning.
Stir in flour 1 minute. Gradually add broth, Worcestershire, ketchup, Dijon, thyme, and gravy salt. Simmer 3 minutes until smooth and lightly thickened.
Transfer hot gravy and hot meatballs to a 4-quart slow cooker. Cover; cook on HIGH 2 hours, stirring gently once, until centers reach 165°F and gravy coats meatballs.
During the final 15 minutes, boil noodles in a large pot of water with noodle-water salt for 7 to 9 minutes, or package time, until tender. Drain; toss with remaining butter.
Divide noodles among 6 bowls; top each with 4 meatballs, mushrooms, and gravy. Serve immediately.