Place the ground chicken, sausage, bread crumbs, garlic, parsley, Pecorino, Parmesan, milk, egg, 1 teaspoon salt, and ½ teaspoon pepper in a bowl and combine gently with a fork.
Form meatballs and fry in a few Tablespoons olive oil to lightly brown.
Add onions at same time and sauté in with the meatballs.
Once golden, remove onions and meatballs to a plate.
Add butter and let melt.
Whisk in flour and cook until golden.
Add warmed broth and cream.
Whisk in mustard.
Bring up to a simmer and add in meatballs.
Cover and cook on low 15 minutes to cook meatballs through.
Garnish with chopped parsley.
Add in egg noodles, if desired