Heat a large pot to a medium heat. Add butter. Once butter melts, add onion and mushroom. Season with 1/4 tsp salt. Saute until slightly softened, about 5 minutes. Add garlic, cook one minute. Stir in flour. Cook 1 minute.
Slowly whisk in beef broth and milk. Add beef boullion, remaining salt, dijon, worcestershire, and nutmeg. Bring to a boil. Add pasta. Bring down to a simmer. Make the meatballs while the pasta cooks in the sauce.
Add chicken, salt, egg, bread, milk, parmesan, onion, garlic, black pepper, and worcestershire to a medium bowl. Use hands to mix the ingredients until well combined. Do not overmix.
Use a small ice cream scoop to form chicken into small balls. Add chicken meatballs to the soup. Continue to cook soup until pasta is aldente and chicken meatballs are cooked through, about 10 minutes. Stir in Greek yogurt.
Season with salt and black pepper.