Soak bread crumbs in milk in a small bowl for 20 minutes. This will remove the grainy texture some meatballs have.
Turn Ninja Foodi to Seart/Sauté on MD:HI. Add oil and onions. Cook onions until translucent.
Mix beef and pork together in a large bowl. Stir onions, bread crumb mixture, eggs, parsley, garlic, salt, black pepper, red pepper flakes, Italian herb seasoning, and Parmesan cheese into meat mixture. Mix well.
Form meat mixture into balls about 1 1/2 inches in diameter.
Turn Ninja Foodi to Sear/Sauté MD:HI. Brown all sides of the meatballs in batches in the Foodi.
These are not cooked all the way through, but you can add to spaghetti and pressure cook, or throw in the oven for about 15-20 minutes At 425°. Or freeze until you decide how to use them.