Preheat oven to 350 degrees Fahrenheit. Line a quarter sheet pan with aluminum foil and spray with cooking spray or olive oil spray. Set it aside.
In a large bowl, combine the ground turkey, minced onion, minced garlic. quick-cooking oats, parmesan cheese, Italian seasoning, and kosher salt.
In a liquid measuring cup, combine the milk and egg. Whisk to combine.
Pour the milk mixture into the turkey mixture. Using clean hands, mix the ingredients until well combined.
Form 1 ¾ inch, round meatballs and arrange them evenly on the prepared sheet pan. You should get 12 meatballs. I made four rows of three, evenly spaced.
In a small bowl, make the egg wash. Add the egg and water to it and whisk to combine. Brush the meatballs with the egg mixture.
Place the meatballs in the oven and bake until they reach an internal temperature of 165 degrees Fahrenheit, about 22 minutes.
Serve with marinara sauce and parmesan cheese.