Heat the oven to 425°F and oil a rimmed sheet. Soak crumbs in milk for 3 minutes, then gently mix with beef, pork, egg, grated onion, meatball salt and pepper. Shape 24 meatballs and bake for 16 to 20 minutes until their centers reach 160°F.
Meanwhile, melt 2 tablespoons butter in a wide saucepan over medium-high heat. Cook mushrooms and sliced onion for 10 to 12 minutes until the mushroom liquid evaporates and the onion begins to brown.
Stir in flour for 1 minute, then gradually add broth, Worcestershire, ketchup, Dijon, thyme and gravy salt. Simmer for 3 minutes until smooth and lightly thickened.
Transfer hot gravy and hot meatballs to a 4-quart slow cooker. Cover and cook on HIGH for 2 hours, stirring gently once, until the centers reach 165°F and the gravy coats them.
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 remaining butter.
Divide noodles among 6 bowls and top each with 4 meatballs, mushrooms and gravy. Serve immediately.